Main Phase Of Magnetic Storm Research Articles

Predicted dependence of the cross polar cap potential saturation on the type of solar wind stream

We study estimations of the cross polar cap potential (CPCP) saturation during magnetic storms induced by various types of solar wind drivers. Using the model of Siscoe–Hill (Hill et al., 1976; Siscoe et al., 2002a,b, 2004; Siscoe, 2011), we evaluate the saturation parameter Q and compare it with the criterion of the CPCP saturation in the main phases of 257 magnetic storms (Dstmin⩽-50nT) induced by the following types of solar wind streams: magnetic clouds (MC), Ejecta, the compressed region, Sheath, before MC (ShMC) and before Ejecta (ShE), corotating interaction regions (CIR), and indeterminate type (IND). Our analysis shows that expected number of events with CPCP saturation is higher for storms induced by ShMC (80%) than for those driven by MC. It is also higher for storms induced by interplanetary coronal mass ejection (ICME) (13.2%) than for storms driven by CIR (3.5%) or by IND (3.5%). The estimated CPCP saturation criterion is satisfied more often for storms initiated by MC (25%) than by Ejecta (2.9%); it is satisfied for 8.6% of magnetic storms induced by the sum of MC and Ejecta, and for 21.5% magnetic storms induced by Sheath before them (sum of ShMC and ShE). These results allow us to conclude that the rate of occurrence of CPCP saturation estimated in the main phase of magnetic storms depends on the type of solar wind driver.

Modeling of the corrected D st * index temporal profile on the main phase of the magnetic storms generated by different types of solar wind

A modeling of the corrected (taking into account the magnetopause currents [9]) Dst* index during the main phase of magnetic storms generated by four types of the solar wind (SW), namely MC (10 storms), CIR (28 storms), Sheath (21 storms), and Ejecta (31 storms), is performed similarly to our previous work on the simple Dst index [8]. The “Catalog of large-scale solar wind phenomena during 1976–2000” ([1], ftp://ftp.iki.rssi.ru/pub/omni/) prepared on the basis of the OMNI database, was used for the identification of SW types. The time behavior of Dst* is approximated by a linear dependence on the integral electric field (sumEy), dynamic pressure (Pd), and fluctuation level (sB) of the interplanetary magnetic field (IMF). Three types of Dst* modeling are performed: (1) by individual values of the approximation coefficients; (2) by approximation coefficients averaged over SW type, and (3) in the same way as in (2) but with allowance for the Dst*-index values preceding the beginning of the main phase of the magnetic storm. The results of modeling the corrected Dst* index are compared to modeling of the usual Dst index. In the conditions of a strong statistical scatter of the approximation coefficients, the use of Dst instead of Dst* insignificantly influences the accuracy of the modeling and correlation coefficient.

Magnetic fluctuations embedded in dipolarization inside geosynchronous orbit and their associated selective acceleration of O+ ions

AbstractWe study magnetic fluctuations embedded in dipolarizations in the inner magnetosphere (a geocentric distance of ≤6.6 RE) and their associated ion flux changes, using the Engineering Test Satellite VIII and Active Magnetospheric Particle Tracer Explorers/CCE satellites. We select seven events of dipolarization that occur during the main phase of magnetic storms having a minimum value of the Dst index less than −40 nT. It is found that (1) all of the dipolarization events are accompanied by strong magnetic fluctuations with the major frequency close to the local O+ gyrofrequency; (2) the magnetic fluctuations appear with significant amplitude in the component nearly parallel to the local magnetic field; (3) the strong flux enhancement is seen in the energy range of 1–10 keV only for O+ ions. In terms of frequency and dominant components of the magnetic fluctuations, they are considered to be excited by the drift‐driven electromagnetic ion cyclotron (EMIC) instability that is recently identified with the linear theory. We perform particle tracing for H+ and O+ ions in the electromagnetic fields modeled by the linear dispersion relation of the drift‐driven EMIC instability. Results show that the O+ ions are accelerated to the energy range of 0.5–5 keV and undergo a significant modification of the spectral shape, while the H+ ions have no clear change of spectral shape, being consistent with the observations. We therefore suggest that the electromagnetic fluctuations associated with the dipolarizations can accelerate O+ ions locally and nonadiabatically in the inner magnetosphere. This selective acceleration of O+ ions may play a role in enhancing the O+ energy density in the storm time ring current.

Dependence of geomagnetic activity during magnetic storms on solar-wind parameters for different types of streams: 4. Simulation for magnetic clouds

In this work, we confirm the possibility of approximating the main phase of a magnetic storm (Dst ≤ −50 nT) caused by magnetic clouds (MCs) with a linear dependence on solar-wind parameters, which are integral electric field sumEy, dynamic pressure Pd, and level of field fluctuations σB. The results show that the main phase of magnetic storm induced by MC is described best by a model with individual values of the main phase approximation coefficients: the correlation coefficient between the measured and model Dst values is 0.99, and the rms deviation is 2.6 nT. The model version with coefficients averaged over all storms describes the main phase much more poorly: the correlation coefficient is 0.65, and the rms deviation is 21.7 nT. A more precise version of the model of the storm main phase induced by MC was developed after introducing corrections that takes into account the history of development of onset of the magnetic-storm main phase: the correlation coefficient is 0.83, and the rms deviation is 15.6 nT. The Dst prediction results during the main phase using the technique suggested are shown for individual magnetic storms as examples.

Modeling the time behavior of the D st index during the main phase of magnetic storms generated by various types of solar wind

Results of modeling the time behavior of the Dst index at the main phase of 93 geomagnetic storms (-250 < Dst ≤ -50 nT) caused by different types of solar wind (SW) streams: magnetic clouds (MC, 10 storms), corotating interaction regions (CIR, 31 storms), the compression region before interplanetary coronal ejec� tions (Sheath before ICME, 21 storms), and "pistons" (Ejecta, 31 storms) are presented. The "Catalog of LargeScale Solar Wind Phenomena during 1976-2000" (ftp://ftp.iki.rssi.ru/pub/omni/) created on the basis of the OMNI database was the initial data for the analysis. The main phase of magnetic storms is approx� imated by a linear dependence on the main parameters of the solar wind: integral electric field sumEy, dynamic pressure Pd, and fluctuation level sB in IMF. For all types of SW, the main phase of magnetic storms is better modeled by individual values of the approximation coefficients: the correlation coefficient is high and the standard deviation between the modeled and measured values of Dst is low. The accuracy of the model in question is higher for storms from MC and is lower by a factor of ~2 for the storms from other types of SW. The version of the model with the approximation coefficients averaged over SW type describes worse varia� tions of the measured D st index: the correlation coefficient is the lowest for the storms caused by MC and the highest for the Sheathand CIRinduced storms. The mo del accuracy is the highest for the storms caused by Ejecta and, for the storms caused by Sheath, is a factor of ~1.42 lower. Addition of corrections for the prehis� tory of the development of the beginning of the main phase of the magnetic storm improves modeling param� eters for all types of interplanetary sources of storms: the correlation coefficient varies within the range from r = 0.81 for the storms caused by Ejecta to r = 0.85 for the storms caused by Sheath. The highest accuracy is for the storms caused by MC. It is, by a factor of ~1.5, lower for the Sheathinduced storms.

Effect of Interplanetary Disturbances and Geomagnetic Activities on Relativistic Electrons at Geosynchronous Orbit

AbstractUsing hourly averaged data of &gt; 2 MeV electron fluxes measured with GOES 7/9/10/11 satellites during 1988–2010, this paper investigates statistically conditions of solar wind and geomagnetic activities during relativistic electrons flux (Fe) enhancements, and describes the dependence of relativistic electrons at geosynchronous orbit (GEO) on local time, magnetic storms. The results are as follows: (1) The local time dependence of GEO relativistic electrons is influenced by solar wind and geomagnetic activities. The noon/midnight electron flux ratio grows with increasing solar wind velocity (Vsw). On conditions of the Dst index above −50 nT, relativistic electrons show regular variation with local time, while during storms with Dst&lt; −50 nT, the maximum electrons flux may not be measured at local noon. During the declining phase of a solar cycle relativistic electron fluxes approaching maximum, Vsw, the Kp index and the cube root of solar wind density (N) show better correlations with electron fluxes after 39∼57 h, 57∼80 h and 12∼24 h, respectively. (2) Before about two years of solar minimum and near the equinoxes, occurrence frequencies of intense relativistic electron flux enhancement events, in which the daily maximum relativistic electron flux Femax ≥ 104 pfu increased, while weaker events with 104 &gt; Femax ≥ 103 pfu exhibited no such solar cycle and seasonal dependences. For most intense events, relativistic electron fluxes begin to increase during main phases of magnetic storms, while for weaker events, relativistic electron fluxes tend to increase during recovery phases. (3) The solar wind density shows as a good indicator of subsequent relativistic electron activities. For most electron flux enhancement events, relativistic electron flux decreased to its minimum after 0∼1 days of the maximum solar wind density, Nmax, while approaching the maximum after 0∼2 days of the minimum solar wind density, Nmin. (4) Above 90% of relativistic electron events occurred on conditions of high‐speed solar wind and geomagnetic disturbances, which are as follows: Vswmax &gt; 516 km/s, Dstmin &lt; −31 nT, Nmin &lt; 2.8 cm−3, Nmax &gt; 14.1 cm−3, Bzmin &lt; −2.9 nT, AEmax &gt; 698 nT. (5) For all magnetic storms, occurrence probability of daily maximum electron flux above 103 pfu after Dstmin, is about 53%, the percent of intense, weak events being 36%, 64%, respectively, which is hardly affected by intensities of storms. During storms, solar wind velocity, density, and the AE index are important indicators for subsequent relativistic electron activities.

Geometry of duskside equatorial current during magnetic storm main phase as deduced from magnetospheric and low-altitude observations

Abstract. We present the results of a coordinated study of the moderate magnetic storm on 22 July 2009. The THEMIS and GOES observations of magnetic field in the inner magnetosphere were complemented by energetic particle observations at low altitude by the six NOAA POES satellites. Observations in the vicinity of geosynchronous orbit revealed a relatively thin (half-thickness of less than 1 RE) and intense current sheet in the dusk MLT sector during the main phase of the storm. The total westward current (integrated along the z-direction) on the duskside at r ~ 6.6 RE was comparable to that in the midnight sector. Such a configuration cannot be adequately described by existing magnetic field models with predefined current systems (error in B &gt; 60 nT). At the same time, low-altitude isotropic boundaries (IB) of &gt; 80 keV protons in the dusk sector were shifted ~ 4° equatorward relative to the IBs in the midnight sector. Both the equatorward IB shift and the current strength on the duskside correlate with the Sym-H* index. These findings imply a close relation between the current intensification and equatorward IB shift in the dusk sector. The analysis of IB dispersion revealed that high-energy IBs (E &gt; 100 keV) always exhibit normal dispersion (i.e., that for pitch angle scattering on curved field lines). Anomalous dispersion is sometimes observed in the low-energy channels (~ 30–100 keV). The maximum occurrence rate of anomalous dispersion was observed during the main phase of the storm in the dusk sector.

Extreme changes in the dayside ionosphere during a Carrington-type magnetic storm

It is shown that during the 30 October 2003 superstorm, dayside O+ ions were uplifted to DMSP altitudes (~850 km). Peak densities were ~9 × 105 cm−3 during the magnetic storm main phase (peak Dst = −390 nT). By comparison the 1–2 September 1859 Carrington magnetic storm (peak Dst estimated at −1760 nT) was considerably stronger. We investigate the impact of this storm on the low- to mid-latitude ionosphere using a modified version of the NRL SAMI2 ionospheric code. It is found that the equatorial region (LAT = 0° ± 15°) is swept free of plasma within 15 min (or less) of storm onset. The plasma is swept to higher altitudes and higher latitudes due to E × B convection associated with the prompt penetration electric field. Equatorial Ionization Anomaly (EIA) O+ density enhancements are found to be located within the broad range of latitudes ~ ± (25°–40°) at ~500–900 km altitudes. Densities within these peaks are ~6 × 106 oxygen ions-cm−3 at ~700 km altitude, approximately +600% quiet time values. The oxygen ions at the top portions (850–1000 km) of uplifted EIAs will cause strong low-altitude satellite drag. Calculations are currently being performed on possible uplift of oxygen neutrals by ion-neutral coupling to understand if there might be further significant satellite drag forces present.

Ion drift in the earth’s inner plasmasphere during magnetospheric disturbances and proton temperature dynamics

Based on the thermal plasma measurements in the Earth’s inner plasmasphere on the INTER-BALL-2 and MAGION-5 satellites it has been indicated that the plasmaspheric ion temperature as a rule decreases during the main phase of magnetic storms; in this case the plasma density increases or remains at the level typical of undisturbed conditions. The physical mechanism by which the ion drift during a magnetic storm results in a temperature decrease is described. It is shown that the third adiabatic invariant also remains in processes with a characteristic time shorter than the period of charged particle drift around the Earth for cold equatorial plasma. The constructed model of the drift shell displacement from the Earth caused by a decrease in the magnetic field in the inner magnetosphere during the development of a magnetic storm satisfactorily describes the decrease in the proton temperature near the equatorial plane.

A Correlative Study of Geomagnetic Storms Associated with Solar Wind and IMF Features During Solar Cycle 23

A geomagnetic storm is a global disturbance in Earth's magnetic field usually occurred due to abnormal conditions in the interplanetary magnetic field (IMF) and solar wind plasma emissions caused by various solar phenomenon. Furthermore the magnitude of these geomagnetic effects largely depend upon the configuration and strength of potentially geo-effective solar/interplanetary features. In the present study the identification of 220 geomagnetic storms associated with disturbance storm time (Dst) decrease of more than -50 nT to -300 nT, have been made, which are observed during 1996-2007, the time period spanning over solar cycle 23. The study is made statistically between the Dst strength (used as an indicator of the geomagnetic activity) and the peak value obtained by solar wind plasma parameters and IMF B as well as its components. We have used the hourly values of Dst index and the wind measurements taken by various satellites. Our results inferred that yearly occurrences of geomagnetic storms are strongly correlated with 11-year sunspot cycle. We observed that IMF B is highly geo-effective during the main phase of magnetic storms, while it more significant at the time of storm peak, which is further contributed by southward component of IMF Bz, substantiating earlier findings. The correlation between Dst and wind velocity is higher, as compared with IMF Bz and ion density. It has been verified that geomagnetic storm intensity is correlated well with the total magnetic field strength of IMF better than with its southward component.

Occurrence of Equatorial Plasma Bubbles during Intense Magnetic Storms

An important issue in low-latitude ionospheric space weather is how magnetic storms affect the generation of equatorial plasma bubbles. In this study, we present the measurements of the ion density and velocity in the evening equatorial ionosphere by the Defense Meteorological Satellite Program (DMSP) satellites during 22 intense magnetic storms. The DMSP measurements show that deep ion density depletions (plasma bubbles) are generated after the interplanetary magnetic field (IMF) turns southward. The time delay between the IMF southward turning and the first DMSP detection of plasma depletions decreases with the minimum value of the IMFBz, the maximum value of the interplanetary electric field (IEF)Ey, and the magnitude of the Dst index. The results of this study provide strong evidence that penetration electric field associated with southward IMF during the main phase of magnetic storms increases the generation of equatorial plasma bubbles in the evening sector.

Statistical study of interplanetary condition effect on geomagnetic storms

Based on the archive OMNI data for the period 1976–2000 an analysis has been made of 798 geomagnetic storms with Dst < −50 nT and their interplanetary sources-large-scale types of the solar wind: CIR (145 magnetic storms), Sheath (96), magnetic clouds MC (62), and Ejecta (161). The remaining 334 magnetic storms have no well-defined sources. For the analysis, we applied the double method of superposed epoch analysis in which the instants of the magnetic storm beginning and minimum of Dst index are taken as reference times. The well-known fact that, independent of the interplanetary source type, the magnetic storm begins in 1–2 h after a southward turn of the IMF (Bz < 0) and both the end of the main phase of a storm and the beginning of its recovery phase are observed in 1–2 h after disappearance of the southward component of the IMF is confirmed. Also confirmed is the result obtained previously that the most efficient generation of magnetic storms is observed for Sheath before MC. On the average parameters Bz and Ey slightly vary between the beginning and end of the main phase of storms (minimum of Dst and Dst* indices), while Dst and Dst* indices decrease monotonically proportionally to integral of Bz and Ey over time. Such a behavior of the indices indicates that the used double method of superposed epoch analysis can be successfully applied in order to study dynamics of the parameters on the main phase of magnetic storms having different duration.

Nightside field-aligned current during the April 6, 2000 superstorm

Using the east-west geomagnetic disturbance fields observed from stations at mid-latitudes, we investigate the characteristics of field-aligned currents on the nightside during the April 6, 2000 superstorm. The results indicate that there is an eastward disturbance on the nightside of Northern Hemisphere during the main phase of magnetic storm, while the interplanetary magnetic field (IMF) is continued southward. The positive disturbances in the postmidnight are larger than that in the premidnight. This suggests that upward field-aligned currents develop on the nightside, when the IMF is directed southward. The peak of upward field-aligned currents is located in postmidnight, and it will be more obvious at substorm expansion. We get the conclusion that both partial ring current and region-2 field-aligned current shift to dusk sector. The upward region-2 current is decreased at substorm onset, and intensified after it. We also investigate the relationship between the upward field-aligned currents on the nightside and the auroral electrojets at high latitudes. It shows a good correspondence between region-2 current and the westward electrojet in the postmidnight. We suppose that they are both associated with the convection electric field.

Are there optical differences between storm-time substorms and isolated substorms?

Abstract. We have performed an extensive analysis of auroral optical events (substorms) that occurred during the development of the main phase of magnetic storms. Using images from the Earth Camera on the Polar spacecraft (Frank et al., 1995), we compared the optical emission features of substorms occurring during 16 expansion phases of magnetic storms with the features of isolated substorms occurring during non-storm times. The comparison used two techniques, visual inspection and statistical comparisons. The comparisons were based on the common characteristics seen in isolated substorms that were initially identified by Akasofu (1964) and quantified by Gjerloev et al. (2008). We find that when auroral activity does occur during main phase development the characteristics of the aurora are very dissimilar to those of the classical isolated substorm. The primary differences include the lack of a surge/bulge, lack of bifurcation of the aurora, much shorter expansion phases, and greater intensities. Since a surge/bulge and bifurcation of the aurora are characteristics of the existence of a substorm current wedge, a key component of the magnetosphere-ionosphere current system during substorms, the lack of this component would indicate that the classical substorm model does not apply to the storm time magnetosphere-ionosphere current system. Rather several of the analyses suggest that the storm-time substorms are associated more closely with the auroral oval, at least spatially, and, therefore, probably with the plasma sheet dynamics during the main phase development. These results then must call into question the widely held assumption that there is no intrinsic difference between storm-time substorms and classical isolated substorms.

F2 layer response to geomagnetic disturbances in Eastern Asia under the low solar activity

In this paper, the peculiarities of ionospheric response to geomagnetic disturbances observed at the decay and minimum of solar activity (SA) in the period 2004–2007 are investigated with respect to different geomagnetic conditions. Data from ionospheric stations and results of total electron content (TEC) measurements made at the network of GPS ground-based receivers located within the latitude–longitude sector (20–70°N, 90–160°Е) are used in this study. Three groups of anomalous ionospheric response to geomagnetic disturbances have been observed during low solar activity. At daytime, the large-scale traveling ionospheric disturbances (LSTIDs) could generally be related to the main phase of magnetic storm. Quasi-two-days wavelike disturbances (WLDs) have been also observed in the main phase independent of the geomagnetic storm intensity. Sharp electron density oscillations of short duration (OSD) occurred in the response to the onset of both main and recovery phases of the magnetic storm in the daytime at middle latitudes. A numerical model for ionosphere–plasmasphere coupling was used to interpret the occurrence of LS TIDs. Results showed that the LSTIDs might be associated with the unexpected lifting of F2 layer to the region with the lower recombination rate by reinforced meridional winds that produces the increase of the electron density in the F2 layer maximum.

Kristian Birkeland's pioneering investigations of geomagnetic disturbances

Abstract. More than 100 years ago Kristian Birkeland (1967–1917) addressed questions that had vexed scientists for centuries. Why do auroras appear overhead while the Earth's magnetic field is disturbed? Are magnetic storms on Earth related to disturbances on the Sun? To answer these questions Birkeland devised terrella simulations, led coordinated campaigns in the Arctic wilderness, and then interpreted his results in the light of Maxwell's synthesis of laws governing electricity and magnetism. After analyzing thousands of magnetograms, he divided disturbances into 3 categories: 1. Polar elementary storms are auroral-latitude disturbances now called substorms. 2. Equatorial perturbations correspond to initial and main phases of magnetic storms. 3. Cyclo-median perturbations reflect enhanced solar-quiet currents on the dayside. He published the first two-cell pattern of electric currents in Earth's upper atmosphere, nearly 30 years before the ionosphere was identified as a separate entity. Birkeland's most enduring contribution toward understanding geomagnetic disturbances flowed from his recognition that field-aligned currents must connect the upper atmosphere with generators in distant space. The existence of field-aligned currents was vigorously debated among scientists for more than 50 years. Birkeland's conjecture profoundly affects present-day understanding of auroral phenomena and global electrodynamics. In 1896, four years after Lord Kelvin rejected suggestions that matter passes between the Sun and Earth, and two years before the electron was discovered, Birkeland proposed current carriers are "electric corpuscles from the Sun" and "the auroras are formed by corpuscular rays drawn in from space, and coming from the Sun". It can be reasonably argued that the year 1896 marks the founding of space plasma physics. Many of Birkeland's insights were rooted in observations made during his terrella experiments, the first attempts to simulate cosmic phenomena within a laboratory. Birkeland's ideas were often misinterpreted or dismissed, but were verified when technology advances allowed instrumented spacecraft to fly in space above the ionosphere.

Spatial-temporal dynamics of auroras during the magnetic storm main phase

The structure and dynamics of auroras in the midnight sector during substorms, which develop during the magnetic storm main phase as compared to the characteristics of a typical auroral substorm, have been studied using the ground-based and satellite observations. It has been found out that a difference from the classical substorm is observed in auroras during the magnetic storm main phase. At the beginning of the storm main phase, the series of pseudobreakups with the most pronounced jump-like motion toward the equator shifts to lower latitudes. The substorm expansion phase can be observed not only as arc jumps to higher latitudes but also as an explosive expansion of a bright diffuse luminosity in all directions. During the magnetic storm main phase, auroras are mainly characterized by the presence of stable extensive rayed structures and by the simultaneous existence of different auroral forms, typical of different substorm phases, in the TV camera field of view.

Prediction of maximal daily average values of relativistic electron fluxes in geostationary orbit during the magnetic storm recovery phase

The relation of the maximal daily average values of the relativistic electron fluxes with an energy higher than 2 MeV, obtained from the measurements on GOES geostationary satellites, during the recovery phase of magnetic storms to the solar wind parameters and magnetospheric activity indices has been considered. The parameters of Pc5 and Pi1 geomagnetic pulsations and the relativistic electron fluxes during the prestorm period and the main phase of magnetic storms have been used together with the traditional indices of geomagnetic activity (AE, Kp, Dst). A simple model for predicting relativistic electron fluxes has been proposed for the first three days of the magnetic storm recovery phase. The predicted fluxes of the outer radiation belt relativistic electrons well correlate with the observed values (R ∼ 0.8–0.9).

Ionospheric disturbances in the years of solar activity minimum and their influence on HF radiowave propagation

The oblique sounding data at the Magadan-Irkutsk and Norilsk-Irkutsk paths together with the vertical sounding at stations located in northeastern Russia were used to analyze ionospheric disturbances in September 2005 and during geophysically active period in December 2006. It is found that during the main phase of magnetic storms, wave disturbances with a period of 2-4 h are registered. These disturbances cause variations in the layer maximum height up to 40-100 km and in the critical frequency up to 1.5-2 MHz. Those variations change substantially values of the maximum observed frequencies (MOF) of the ionospheric radio channel at the paths considered. Such wave disturbances can be caused by generation of AGWs in the auroral zone and their propagation to equatorial latitudes.

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.