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The article presents the results of a studying detection of the sudden commencement (SC) and main impulse (MI) of a magnetic storm as a function of the geographic coordinates of magnetic observatories and Universal Time, using modern data with second time resolution. The analysis was carried out for two events in which an interplanetary shock wave impacting the magnetosphere was associated with interplanetary coronal mass ejections (CMEs) with sources in different hemispheres of the Sun. The authors propose an approach to determine the time points of SC and MI detection. It is concluded that the SC and MI detection times may differ by several seconds to more than a minute at magnetic observatories located at different geographic latitudes and longitudes. For the studied events, the graphs of SC and MI detection as functions of the geographic coordinates of magnetic observatories and Universal Time revealed trends according to which, on average, the higher station the latitude, the later SC and MI are detected at the station. [ABSTRACT FROM AUTHOR]

Here we studied the planetary features of the spatiotemporal distribution of ionospheric electrojets recorded in the onset of a substorm and in time on the activity maximum of three very intense substorms (with an AL-index from –1200 to –1700 nT) observed during the main phase of the strong magnetic storm on March 23−24, 2023. We have analyzed the substorms by applying the global maps of the planetary distribution of high-latitude ionospheric currents, compiled from simultaneous magnetic measurements on 66 low-orbit satellites of the AMPERE project, as well as ground-based magnetograms from the Scandinavian IMAGE profile and mid-latitude IZMIRAN stations located in the same longitudinal region. It was established that the onset of all the studied substorms on the IMAGE meridian was accompanied by the development of a nighttime current vortex with clockwise rotation, which is an indicator of an increase in downward field-aligned currents. The ground-based mid-latitude observations at the IZMIRAN station network confirmed that the center of the current wedge of the substorm was located in the nighttime sector significantly east of the IMAGE meridian. In the time of the substorm intensity maximum, a similar but more extensive current vortex was observed in the morning sector, which is probably typical of intense substorms. [ABSTRACT FROM AUTHOR]

We analyzed 214 events of 'polar' substorms on the Scandinavian meridian IMAGE, i.e., substorms recorded by magnetometers located at geomagnetic latitudes above ~70° MLAT at 1900−0200 MLT during a magnetically quiet time in the absence of negative magnetic bays at lower latitudes. The Harang discontinuity, which separates the westward and eastward electrojets by latitude, is a typical structure for the indicated MLT sector of the high-latitude ionosphere. The global distribution of ionospheric electrojets and the location of the Harang discontinuity during development of 'polar' substorms were studied using the maps compiled from the results of spherical harmonic analysis of magnetic measurements on 66 simultaneous ionospheric communications satellites of the AMPERE project. Based on analysis of these maps, it is shown that the instantaneous location of the equatorial boundary of the ionospheric current of a 'polar' substorm determines the instantaneous location of the polar boundary of the Harang discontinuity, and the polar boundary of the eastward electrojet determines its equatorial boundary. It has been established that the appearance of 90% of 'polar' substorms is observed simultaneously with increasing planetary substorm activity according to the AL-index and development of a magnetospheric substorm in the postmidnight sector. At the same time, the development of evening 'polar' substorms is associated with the formation of near-midnight magnetic vortices at geomagnetic latitudes of ~70° MLAT (near the "nose" of the Harang discontinuity), indicating a sharp local enhancement of the field-aligned currents. This leads to the formation of a new substorm in the evening sector of near-polar latitudes, called a 'polar' substorm with typical features of the onset of a substorm (Pi2 geomagnetic pulsation bursts, sudden onset of the substorm close to the equatorial boundary of the contracted oval (the development of a "substorm current wedge", etc.) [ABSTRACT FROM AUTHOR]

The features of the geomagnetic effect of the approach of an interplanetary magnetic cloud to the Earth's magnetosphere during the recovery phase of a strong magnetic storm (June 21–26, 2015) are studied. On the Earth's surface, the greatest geomagnetic effects were recorded when the leading edge of the magnetic cloud approached the Earth's magnetosphere, where large alternating variations in the Bz and By components of the IMF were observed. Large (up to 1000 nT), bay-like magnetic disturbances were observed in the near-noon sector of the circumpolar latitudes, presumably in the region of the projection of the daytime polar cusp. It is shown that the sign of the high-latitude magnetic bay was controlled by the sign of the By component of the IMF and did not depend on the direction of the Bz component of the IMF. The planetary, high-latitude geomagnetic activity and the distribution of large-scale, field aligned electric currents calculated from magnetic observations were studied based on simultaneous magnetic registration on 66 ionospheric communication satellites (project AMPERE). It was found that, in the case of the development of a negative magnetic bay (the western electrojet), the downward field aligned electric currents in the midday sector were observed to be more polar than the upward currents. For a positive bay (the eastern electrojet), the upward currents were more polar than the downward ones. It is shown that an abrupt change in the sign of the By components of the IMF led not only to a sharp change in the direction of the ionospheric current, but also to its movement in latitude. [ABSTRACT FROM AUTHOR]

The behavior of geomagnetic disturbances during a moderate CIR storm, the most intense magnetic storm of 2009 in the solar activity minimum are considered. It is found that the substorm activity during the main phase of the storm was observed not in the nighttime but in the morning sector (0700–0900 MLT) of auroral geomagnetic latitudes (65°–67°) and was accompanied by extremely large (up to 1000 nT) geomagnetic pulsations of the Рс5 range, evidently of a non-resonant nature; the amplitude of the Y component of waves considerably exceeded that of the Х component. In the recovery phase of the magnetic storm, the tangential discontinuity in the IMF caused a substorm observed near the polar boundary of the auroral oval in the global scale in the longitude, from the evening sector to midday hours. In the daytime sector, the magnetic bay was observed several minutes later than in the nighttime sector and rapidly moved to the pole, which can be interpreted as the motion to the pole of the boundary between closed and open geomagnetic field lines and contraction of the auroral oval. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The features of daytime high-latitude geomagnetic variations and geomagnetic pulsations in the Рс5 range during the recent, large, two-stage magnetic storm of September 7-8, 2017 are studied. The discussed disturbances were observed at the recovery phase of the first stage of the storm after the interplanetary magnetic field (IMF) turned northward. It is shown that the large sign-alternating variations in Ву and Bz components of the IMF caused intense geomagnetic disturbances up to 300-400 nT with a quasi-period of ~20 min in the daytime sector of polar latitudes, probably in the region of the daytime polar cusp. These disturbances may have reflected quasi-period motions of the daytime magnetopause and may have resulted from nonlinear transformation of the variations in the interplanaterary magnetic field in the magnetosheath or in the magnetospheric entry layers. The appearance of high-latitude long-period variations was accompanied by the excitation of bursts (wave packets) of geomagnetic Pc5 pulsations. The onset of Pc5 pulsation bursts often coincided with a sudden northward turn of the IMF. It was discovered for the first time that the development of a “daytime polar substorm,” i.e., a negative magnetic bay in the daytime sector of polar latitudes, led to a sudden termination of the generation of geomagnetic Pc5 pulsations over the entire latitude range in which these oscillations were recorded before the appearance of the daytime bay. [ABSTRACT FROM AUTHOR]

Polar substorms include substorms observed at geomagnetic latitudes above 70° MLAT in the absence of simultaneous negative magnetic bays at lower latitudes, that is, substorms on the compressed contracted auroral oval. The general morphological features of polar substorms are considered based on the example of individual events registered on Svalbard arch. It is shown that polar substorms, like "classical" substorms, are characterized by the formation of a substorm current wedge and a steplike movement to the pole after the onset of a substorm, generation of Pi2 geomagnetic pulsations, and an increase of the PC-index of the polar cap before the onset of the substorm. At the same time, there are certain differences between polar substorms and "classical" substorms; namely, they start on more distant L-shells, develop in the region of a contracted auroral oval, occur at earlier pre-midnight hours, and generate only at low solar wind speeds and weakly disturbed geomagnetic conditions. It has been suggested that polar substorms may be a specific type of "classical" substorms that develop in the evening sector under magnetically quiet or weakly disturbed conditions when the auroral oval is concracted. The source of polar substorms may also be a local intensification of previously existing substorms in the post-midnight sector. [ABSTRACT FROM AUTHOR]

The development of two supersubstorms (i.e., very intense substorms with an amplitude of more than 2000 nT) recorded in the main phase of two consecutive strong magnetic storms with maxima at ~0100 UT (Dst ~ –150 nT) and ~1300 UT (Dst ~ –115 nT) on September 8, 2017, is analyzed. Data from the SuperMAG global magnetometers network and the Scandinavian profile of IMAGE stations are used. Analysis of spatial distribution maps of ionospheric equivalent electric currents on the Scandinavian meridian derived according to the MIRACLE model and global maps of magnetic field vectors obtained from SuperMAG observations makes it possible to obtain the spatial distribution of planetary-scale disturbances. Both supersubstorms were characterized not only by strong nighttime disturbances at auroral latitudes (~–3600 nT and ~–2600 nT) but also by the simultaneous development of daytime magnetic bays at polar latitudes with amplitudes of ~–1000 nT and ~–400 nT, respectively. We assume that the daytime polar disturbances observed simultaneously with the supersubstorms can result from the pull of the westward ionospheric current to the dayside. Our observations support the assumption that the westward electrojet during the supersubstorm develops on a global scale from the evening to the dayside. [ABSTRACT FROM AUTHOR]

We have studied the geomagnetic conditions during the occurrence of auroral-hiss VLF emissions on the Earth's surface in the form of noise bursts at frequencies above 5–6 kHz. The study is based on an analysis of auroral-hiss observations at auroral latitudes in northern Finland at Kannuslehto station (KAN, L ~ 5.5) during the winter campaigns of 2013–2018. It is shown that auroral-hiss bursts are most often observed in the interval of 20–01 MLT under low geomagnetic activity (Kp < 3). It has been found that the bursts are typical for the growth phase of a magnetospheric substorm; the bursts cease abruptly at the onset of the substorm (the break-up of auroras), which is apparently due to a drastic increase in the absorption of VLF waves in the ionosphere. The bursts are often accompanied by the generation of geomagnetic Pi2 pulsations. VLF observations have shown that auroral-hiss bursts are not observed on the Earth's surface in the main phase of magnetic storms; however, they are typical of the recovery phase. It has been found from model data that, during the occurrence of auroral hiss, KAN station is usually projected to the near-equatorial region of the auroral oval or to the zone of diffuse precipitation of more energetic electrons, i.e., to lower latitudes than the typical position of visible auroras during that time. [ABSTRACT FROM AUTHOR]