Night Sector Research Articles

The dynamics of quasi-periodic ripples in the high-latitude F-region

This study extends the investigation of quasi-periodic ripples in the F-region electron content reported by Birch and Hargreaves (2020a) to determine their flow velocity and wavelength, using data from the EISCAT Svalbard radars (the 42m antenna being field-aligned and the 32m scanning in a circular pattern at a fixed angle from the field). The ripples were extracted from the 42m electron content between altitudes 213 and 389 km in the noon and midnight sectors using a bandpass filter which reduces the noise component while removing long-term trends. These ripples were found to have an average periodicity of about 21 min in the noon sector and 27 min at night. Using the 32m line-of-sight velocity data, they were also found to propagate in the noon sector at about 467 m/s in a westerly direction with a wavelength of 581 km, and in the night sector at about 283 m/s in a southerly direction with a wavelength of 454 km. The directions compare favourably with modelled plasma flows from the SuperDARN network of coherent scatter radars.

The relevance of local magnetic records when using extreme space weather events as benchmarks

Space weather indices introduced for scientific purposes are commonly used to quantify operational nowcast of the geospace state during extreme space weather events. Some indices, such as the Disturbance storm time (Dst) index, have been applied to situations for which they are not originally intended. This raises a question about suitability as a space weather benchmark. In analysing historical records for different magnetometers at low- and mid-latitude, we find periods with longitudinal asymmetry in magnetic response that suggest important signals from individual magnetometers are being averaged out of the Dst record. This asymmetry develops as a double spike in the H-component: one negative in the observatories in the day sector and one positive in the observatories in the night sector. These spikes develop in short-time (about 2 h) and pose a potential hazardous effect for users affected by space weather. The results from historical events have been reinforced with the systematic study of magnetic records during extreme events (Dst ≤ −200 nT and AL ≤ −2000 nT) in the period 1998–2017 from six magnetic observatories at about 40° magnetic latitude. Moreover, we show that the largest asymmetries take place during the early main phase and are recorded in narrow local time sectors. An important outcome of these results is that space weather benchmarks should be based on local records instead of the commonly used global indices. This action improves two important aspects of space weather: the assessment of historical extreme events and that of the needs of users.

Effect of High-Intensity Electron and Proton Fluxes on a Low-Latitude Ionosphere

Satellite experiments at low latitudes have shown that energetic (tens of keV) electrons and protons can penetrate from the Earth’s radiation belt near the equator into the ionosphere. The effect of the fluxes of these particles on the upper atmosphere and ionosphere remains understudied. In this study, local variations in the concentration of ionospheric ions in the regions of injection of electrons and protons with energies higher than 30 keV into the low-latitude ionosphere are studied using the magnetic storm of July 22, 2009, as an example. The relationship between an increase in the concentration in the F layer in the morning sector and additional ionization by fast electrons is shown. The ionospheric inhomogeneities observed in the night sector in the form of strong concentration fluctuations are caused by the action of protons. The solar-cyclic and annual variations in the frequency of the penetration of electrons and protons from the Earth’s radiation belt into the low-latitude ionosphere for the period from 1998 to 2019 are analyzed.

On the source region and orientations of nightside auroral arcs

Using large data base of stable auroral arcs observed at Gillam we identified those passes of POES spacecraft which were magnetically conjugate to these arcs. We studied statistically arc locations with respect to the energetic particle isotropy boundaries (IBs), which are the well-known markers of the magnetotail regions. In the dusk and night sectors we confirm previous finding that arcs tend to appear close to or poleward of the electron isotropy boundary (IBe), indicating the location of the arcs in the small BZ (large magnetic field curvature) region of the tail current sheet. Different from that, in the dawn sector significant fraction of arcs appears deeper in the magnetosphere, between electron and proton IBs, in the transition region between the current sheet and dipole-like magnetosphere. Addressing the orientations of stable arcs we use the empirical magnetospheric models to demonstrate that nearly sun-aligned plasma sheet structures in the tail are significantly rotated toward azimuthal alignment when mapped from the tail equator to the ionosphere. We discuss how the flow braking/rotation can provide (in direct or secondary/way) the E-W aligned arcs. Based on these results we argue that a significant fraction of the highlatitude stable auroral arcs could be formed by the fast flow convection channels (BBFs); particularly the observed dawn-dusk asymmetry in the relative location of arcs and isotropy boundary can be contributed by this mechanism.

Magnetospheric “Penetration” of IMF Viewed Through the Lens of an Empirical RBF Modeling

AbstractThe spiral structure of the interplanetary magnetic field (IMF) is known to induce intramagnetospheric azimuthal magnetic field , which strongly correlates with the IMF . We reconstruct this effect for the first time in 3‐D, using a large set of data taken in the near/inner magnetosphere and a flexible magnetic field model based on expansions in radial basis functions (RBF). The RBF model serves here as a magnifying glass with tunable resolution, focused on the specific region of interest. In this study, we used it to explore the IMF‐induced both on a global scale (i.e., for the entire range of local times) and in the night sector only, to better visualize details in the region with the strongest “penetration” magnitude. The induced was found to maximize on the nightside at distances 10–12 , where it concentrates around the solar‐magnetic equator and bifurcates into a pair of peaks located in predawn and postdusk sectors. The “penetration” is associated with the IMF‐induced asymmetry of field‐aligned currents at the plasma sheet boundary. Even on a statistical level, the peak values of the induced can substantially exceed the external IMF . The effect is significantly stronger under southward IMF conditions and grows with increasing geodipole tilt angle.

Magnetic local time asymmetries in precipitating electron and proton populations with and without substorm activity

Abstract. The magnetic local time (MLT) dependence of electron (0.15–300 keV) and proton (0.15–6900 keV) precipitation into the atmosphere based on National Oceanic and Atmospheric Administration POES and METOP satellite data during 2001–2008 was described. Using modified APEX coordinates the influence of particle energy, substorm activity and geomagnetic disturbance on the MLT flux distribution was statistically analysed. Some of the findings are the following. a. Substorms mostly increase particle precipitation in the night sector by about factor 2–4, but can also reduce it in the day sector.b. MLT dependence can be assigned to particles entering the magnetosphere at the cusp region and magnetospheric particles in combination with energy-specific drifts (in agreement with Newell et al., 2009).c. MLT flux differences of up to 2 orders of magnitude have been identified inside the auroral oval during geomagnetically disturbed conditions. The novelty here is the comprehensive coverage of energy bands and the focus on asymmetry.d. The maximum flux asymmetry ratio depends on particle energy, decreasing with Kp for low energetic particles and increasing with Kp for higher energy electrons, while high energy protons show a more complex dependency. While some aspects may already have been known, the quantification of the flux asymmetry sheds new light on MLT variation.

The Magnetic Storm of August 25–26, 2018: Dayside High Latitude Geomagnetic Variations and Pulsations

The features of daytime high latitude geomagnetic disturbances and geomagnetic pulsations during the recent strong magnetic storm on August 25–26, 2018, which occurred at the end of the decline phase of the 24th solar activity cycle with a very low level of solar flare activity, are considered. As a rule, during this phase of the solar activity cycle, magnetic storms are caused by high-speed solar wind flows from coronal holes; however, the magnetic storms during the decay of the 24th cycle were caused by coronal mass ejections (CMEs). It was shown that, despite very weak disturbances on the Sun and a low solar wind speed, a rather strong magnetic storm occurred in the Earth’s magnetosphere in August 2018 (Dst = –171 nT). The storm SC with a slight (~25 nT) jump in the Dst index caused quite intense daytime geomagnetic pulsations ipcl at the latitudes of the possible position of the daytime polar cusp. A feature of the recovery phase of this storm was the development of a magnetosphere substorm on a global scale, i.e., the appearance of a negative magnetic bay recorded simultaneously in the auroral night sector and in the polar latitudes of the day sector. A possible interpretation is considered.

Lightning Contribution to Overall Whistler Mode Wave Intensities in the Plasmasphere

AbstractElectromagnetic waves generated by lightning propagate into the plasmasphere as dispersed whistlers. They can therefore influence the overall wave intensity in space, which, in turn, is important for dynamics of the Van Allen radiation belts. We analyze spacecraft measurements in low‐Earth orbit as well as in high‐altitude equatorial region, together with a ground‐based estimate of lightning activity. We accumulate wave intensities when the spacecraft are magnetically connected to thunderstorms and compare them with measurements obtained when thunderstorms are absent. We show that strong lightning activity substantially affects the wave intensity in a wide range of L‐shells and altitudes. The effect is observed mainly between 500 Hz and 4 kHz, but its frequency range strongly varies with L‐shell, extending up to 12 kHz for L lower than 3. The effect is stronger in the afternoon, evening, and night sectors, consistent with more lightning and easier wave propagation through the ionosphere.

Studying 630 nm atomic oxygen emission sources during strong magnetic storms in the night mid-latitude ionosphere

We analyze significant increases in 630 nm atomic oxygen night emissions during very strong geomagnetic storms, using optical measurements, theoretical modeling, and magnetogram inversion technique (MIT) data. It is shown that during strong magnetic storms when electron precipitation equatorial boundary at the night sector expands up to ~40°, the interaction of energetic electron flux with thermospheric components may cause extreme increases in the 630 nm emission intensity. Model calculations of the red line intensity show good agreement with observational data. Using the November 20, 2003 magnetic storm as an example, we have found that oxygen atom collisions with thermal Maxwell and superthermal electrons make a major contribution to the integral emission intensity. Thermospheric density variations during the magnetic storm significantly affect the red line generation.

Изучение источников эмиссии атомарного кислорода 630 нм во время сильных магнитных бурь в ночной среднеширотной ионосфере

We analyze significant increases in 630 nm atomic oxygen night emissions during very strong geomagnetic storms, using optical measurements, theoretical modeling, and magnetogram inversion technique (MIT) data. It is shown that during strong magnetic storms when electron precipitation equatorial boundary at the night sector expands up to ~40°, the interaction of energetic electron flux with thermospheric components may cause extreme increases in the 630 nm emission intensity. Model calculations of the red line intensity show good agreement with observational data. Using the November 20, 2003 magnetic storm as an example, we have found that oxygen atom collisions with thermal Maxwell and superthermal electrons make a major contribution to the integral emission intensity. Thermospheric density variations during the magnetic storm significantly affect the red line generation.

Time Scales for Electron Quasi‐linear Diffusion by Lower‐Band Chorus Waves: The Effects of ωpe/Ωce Dependence on Geomagnetic Activity

AbstractElectron scattering by chorus waves is an important mechanism that can lead to fast electron acceleration and loss in the outer radiation belt. Making use of Van Allen Probes measurements, we present the first statistical survey of megaelectron volt electron pitch angle and energy quasi‐linear diffusion rates by chorus waves as a function of L‐shell, local time, and AE index, taking into account the local electron plasma frequency to gyrofrequency ratio ωpe/Ωce, chorus wave frequency, and resonance wave amplitude. We demonstrate that during disturbed periods, ωpe/Ωce strongly decreases in the night sector, leading to a faster electron loss but also a much faster electron energization in two distinct regions just above the plasmapause and at L ~ 3.5–5.5. Spatiotemporal variations of ωpe/Ωce with AE shape the evolution of electron energization in the outer belt, sometimes leading to very short time scales for quasi‐linear megaelectron volt electron acceleration in agreement with Van Allen Probes observations.

Equatorial Disturbance Dynamo Vertical Plasma Drifts Over Jicamarca: Bimonthly and Solar Cycle Dependence

AbstractWe use extensive incoherent scatter radar observations from the Jicamarca Radio Observatory to study the local time and bimonthly dependence of the equatorial disturbance dynamo vertical plasma drifts on solar flux and geomagnetic activity. We show that the daytime disturbance drifts have generally small magnitudes with largest values before noon and an apparent annual variation. Near dusk, they are downward throughout the year with largest values during the equinoxes and smallest during June solstice. These downward drifts increase strongly with solar flux and shift to later local times. They also increase with increasing geomagnetically active conditions with no apparent local time shift. The equinoctial evening downward disturbance drifts are larger during the autumnal equinox than during the vernal equinox. The nighttime disturbance drifts are upward and have small seasonal and solar cycle dependence but increase strongly with geomagnetic activity, particularly in the late night sector. Our results are in general agreement with those from previous theoretical and experimental studies, except near dusk where our results show much stronger seasonal and solar cycle dependence.

Influence of Substorm Activity on the Formation of Ultra Low Frequency Noise Emissions in the Frequency Range of 0–7 Hz

The results of the study of simultaneous observations of midlatitude ultra low frequency (ULF) noise emissions in the frequency range of 0–7 Hz and disturbances in the night sector of an auroral oval are presented. Data from observations of the magnetic field at the midlatitude Borok Observatory (L = 2.8) and 1-min data of the AL index are used for the analysis. It is shown that two kinds of ULF noise emission are observed simultaneously in the hertz range primarily in the summer season: one looks like a diffuse spot, and the other one is characterized by the occurrence of spectral resonance structures determined by the influence of the ionospheric Alfven resonator. Diffuse spots are recorded mostly in the evening sector of the magnetosphere, and they precede the observation of spectral resonance structures, which are detected primarily at about midnight. Comparison of the observation intervals for diffuse spots with the dynamics of the AL index showed that diffuse spots form during substorm activity on the night side of the Earth in 80% of cases. It is established that the interval between the onset of substorm disturbances and the onset of the observation of diffuse spots is ∼60 min in most cases. It is assumed that the formation of diffuse spots is related to characteristics of the dynamics of protons injected from the magnetospheric tail in substorms and to the appearance of plasmaspheric plumes in the evening sector.

The solar wind-magnetosphere coupling and daytime disturbance electric fields in equatorial ionosphere during consecutive recurrent geomagnetic storms

This paper investigates evolutions of geomagnetic and equatorial electrodynamic responses during three consecutive 27-day recurrent geomagnetic storms (RGSs) in 2007 by using the solar wind plasma, geomagnetic indices, and magnetometer data. The RGSs are classified as corotating interaction regions (CIR)-induced storms and High Intensity Long Duration Continuous AE Activity (HILDCAA). The RGSs show quasi-stable and diverge variabilities in storm-times that critically control the long-lasting and complex electrodynamic responses in equatorial ionosphere, particularly in CIR-storms. The correlations of Auroral Electrojet (AE)/Polar Cap (PC) indices are moderate for CIRs and HILDCAAs during local summer, which suggest to less effective coupling and/or different sources of PC and AE. The correlations of reconnection electric field with AE and Fourier analysis of the north-south component of the interplanetary magnetic field with AE reveal that the solar wind-magnetosphere-ionosphere coupling is more effective in the HILDCAAs than in the CIRs. Results indicate that disturbance electric fields are closely related to storm time evolution of the RGSs and are seasonal dependent. Disturbance dynamo electric fields (DDEFs) are related to each group of HSSs during HILDCAA periods. Justification of storm-time effect or seasonal effects is done. The substantial DDEFs are caused by the storm time effects of enhanced thermospheric wind due to the summer-to-winter winds in the late night sector. DDEFs are effective particularly in the morning-to-prenoon time in the summer, while they are less effective in equinoctial months due to the symmetric meridional winds and in winter months due to restriction of the equatorward motion.

Explicit IMF By Dependence in High‐Latitude Geomagnetic Activity

AbstractThe interaction of the solar wind with the Earth's magnetic field produces geomagnetic activity, which is critically dependent on the orientation of the interplanetary magnetic field (IMF). Most solar wind coupling functions quantify this dependence on the IMF orientation with the so‐called IMF clock angle in a way, which is symmetric with respect to the sign of the By component. However, recent studies have suggested that the sign of By is an additional, independent driver of high‐latitude geomagnetic activity, leading to higher (weaker) geomagnetic activity in Northern Hemisphere (NH) winter for By > 0 (By<0). In this paper we quantify the size of this explicit By effect with respect to the solar wind coupling function, both for northern and southern high‐latitude geomagnetic activity. We show that high‐latitude geomagnetic activity is significantly (by about 40%–50%) suppressed for By < 0 in NH winter and for By > 0 in Southern Hemisphere winter. When averaged over all months, high‐latitude geomagnetic activity in NH is about 12% weaker for By < 0 than for By > 0. The By effect affects the westward electrojet strongly, but hardly at all the eastward electrojet. We also show that the suppression of the westward electrojet in NH during By < 0 maximizes when the Earth's dipole axis points toward the night sector, that is, when the auroral region is maximally in darkness.

Relationship between PC index and magnetospheric field-aligned currents measured by Swarm satellites

The relationship between the magnetospheric field-aligned currents (FAC) monitored by the Swarm satellites and the magnetic activity PC index (which is a proxy of the solar wind energy incoming into the magnetosphere) is examined. It is shown that current intensities measured in the R1 and R2 FAC layers at the poleward and equtorward boundaries of the auroral oval are well correlated, the R2 currents being evidently secondary in relation to R1 currents and correlation in the dawn and dusk oval sectors being better than in the noon and night sectors. There is evident relationship between the PC index and the intensity of field-aligned currents in the R1 dawn and dusk layers: increase of FAC intensity in the course of substorm development is accompanied by increasing the PC index values. Correlation between PC and FAC intensities in the R2 dawn and dusk layers is also observed, but it is much weaker. No correlation is observed between PC and field-aligned currents in the midnight as well as in the noon sectors ahead of the substorm expansion phase. The results are indicative of the R1 field-aligned currents as a driver of the polar cap magnetic activity (PC index) and currents in the R2 layer.

Impact of Sudden Stratospheric Warming of 2009 on the Equatorial and Low‐Latitude Ionosphere of the Indian Longitudes: A Case Study

AbstractUsing the equatorial electrojet (EEJ)‐induced surface magnetic field and total electron content (TEC) measurements, we investigated the impact of the sudden stratospheric warming (SSW) of January 2009 on the equatorial electrodynamics and low‐latitude ionosphere over the Indian longitudes. Results indicate that the intensity of EEJ and the TEC over low latitudes (extending up to 30°N) exhibit significant perturbations during and after the SSW peak. One of the interesting features is the deviation of EEJ and TEC from the normal quiet time behavior well before the onset of the SSW. This is found to coincide with the beginning of enhanced planetary wave (PW) activity over high latitudes. The substantial amplification of the semidiurnal perturbation after the SSW peak is seen to be coinciding with the onset of new and full moons. The response of TEC to SSW is found to be latitude dependent as the near‐equatorial (NE) stations show the semidiurnal perturbation only after the SSW peak. Another notable feature is the presence of reduced ionization in the night sector over the NE and low‐latitude regions, appearing as an “ionization hole,” well after the SSW peak. The investigation revealed the existence of a quasi 16 day wave in the TEC over low latitudes similar to the one present in the EEJ strength. These results have been discussed in the light of changes in the dynamical background because of enhanced PW activity during SSW, which creates favorable conditions for the amplification of lunar tides, and their subsequent interaction with the lower thermospheric tidal fields.

Response of the night aurora to a negative sudden impulse

Data from the meridian scanning photometers of the NORSTAR network and all-sky cameras of the THEMIS network were used for a detailed study of the response of night auroras to the sharp decrease of the solar wind dynamic pressure on September 28, 2009. The decrease in dynamic pressure was accompanied by a corresponding depression of the magnetic field in the SYM-H index and the origin of a negative sudden impulse (SI) with a duration of 5–8 min and amplitude of 150–200 nT in the horizontal component of the magnetic field at stations of the night sector of the auroral zone. The magnetic impulse was preceded by a long calm magnetic period, although the IMF Bz-component was negative for ~1.5 hour before the SI–. The commencement of the SI–, which was determined by variations in the magnetic field at ~0650 UT, was accompanied by a sharp increase in the intensity of discrete forms of polar auroras in the midnight sector of the auroral zone and their fast propagation to the pole. Approximately 6–8 min after the SI–, the auroral intensity in the emissions, which were excited by the fluxes of precipitated electrons and protons, quickly began to decrease in the night sector. Analysis of the optical observations showed the two-stage character of the response of the night auroras to the SI– in the considered event: first, fast movement of the discrete aurora forms to the pole with a significant increase in their intensity, and a further fast decrease in auroral intensity with a delay of ~6–8 min relative to the SI–. The possible reasons for such aurora behavior are discussed.

THEMIS observations and riometry: A data comparison with a view to proxy and prediction

A comparison has been made between radio absorption events observed by riometer at selected times of day and bursts of electrons observed in the midnight sector by THEMIS satellites. The correlation is found to be good for absorption in the noon and midnight sectors but poor around dawn and dusk. For noon and midnight the absorption can be estimated from the THEMIS electron flux to better than a factor of 2 in most cases. In the noon sector the absorption follows the THEMIS event by about 30min on average (though with considerable variation from case to case), but by night the absorption precedes the electron flux by about 8min on average. Thus, the flux at THEMIS can be predicted from the absorption in the night sector, the accuracy being better than a factor of 3 in most cases. The flux observed also depends on the location of the satellite, reducing with increasing distance down the tail. It is estimated that the source of the tail events observed in this study was at about 6Re, and comparisons are made with the established general pattern of the substorm in auroral absorption.

Empirical model of lower band chorus wave distribution in the outer radiation belt

AbstractAccurate modeling of wave‐particle interactions in the radiation belts requires detailed information on wave amplitudes and wave‐normal angular distributions over L shells, magnetic latitudes, magnetic local times, and for various geomagnetic activity conditions. In this work, we develop a new and comprehensive parametric model of VLF chorus waves amplitudes and obliqueness in the outer radiation belt using statistics of VLF measurements performed in the chorus frequency range during 10 years (2001–2010) aboard the Cluster spacecraft. We used data from the Spatio‐Temporal Analysis of Field Fluctuations‐Spectrum Analyzer experiment, which spans a total frequency range from 8 Hz to 4 kHz. The statistical model is presented in the form of an analytical function of latitude and Kp (or Dst) index for day and night sectors of the magnetosphere and for two ranges of L shells above the plasmapause, from L = 4 to 5 and from L = 5 to 7. This model can be directly applied for numerical calculations of charged particle pitch angle and energy diffusion coefficients in the outer radiation belt, allowing to study with unprecedented detail their statistical properties as well as their important spatiotemporal variations with geomagnetic activity.