Number Of Substorms Research Articles

Isolated substorms according to magnetic measurements at Tixie during minimum solar activity

A catalog of isolated substorms in 2016–2020 has been compiled from data on the H component of the geomagnetic field, obtained at Tixie. From the catalog data, it has been found that during this period changes in the number of substorms and the number of sunspots are well approximated by quadratic functions with minima at the end of 2017 and in the middle of 2019 respectively; during the year, disturbances more often occurred during solstice; within 24 hours, substorms more often occurred at local midnight. The intensity and duration of substorm disturbances, the duration of their expansion phase do not show a noticeable dependence on the time of occurrence; however, from average values of these parameters in hourly ranges, it has been found that the intensity takes lower values around 0–3 MLT; in the midnight sector, the duration of disturbances and the duration of their expansion phase are shorter than those in the dawn sector. Compared to the data from mid-latitude stations [Chu et al., 2015], the average duration of substorms and the duration of their expansion phase are longer.

Изолированные суббури по данным магнитных измерений в Тикси в период минимальной солнечной активности

A catalog of isolated substorms in 2016–2020 has been compiled from data on the H component of the geomagnetic field, obtained at Tixie. From the catalog data, it has been found that during this period changes in the number of substorms and the number of sunspots are well approximated by quadratic functions with minima at the end of 2017 and in the middle of 2019 respectively; during the year, disturbances more often occurred during solstice; within 24 hours, substorms more often occurred at local midnight. The intensity and duration of substorm disturbances, the duration of their expansion phase do not show a noticeable dependence on the time of occurrence; however, from average values of these parameters in hourly ranges, it has been found that the intensity takes lower values around 0–3 MLT; in the midnight sector, the duration of disturbances and the duration of their expansion phase are shorter than those in the dawn sector. Compared to the data from mid-latitude stations [Chu et al., 2015], the average duration of substorms and the duration of their expansion phase are longer.

Examination of Radiation Belt Dynamics During Substorm Clusters: Activity Drivers and Dependencies of Trapped Flux Enhancements

AbstractDynamical variations of radiation belt trapped electron fluxes are examined to better understand the variability of enhancements linked to substorm clusters. Analysis is undertaken using the Substorm Onsets and Phases from Indices of the Electrojet substorm cluster algorithm for event detection. Observations from low earth orbit are complemented by additional measurements from medium earth orbit to allow a major expansion in the energy range considered, from medium energy energetic electrons up to ultra‐relativistic electrons. The number of substorms identified inside a cluster does not depend strongly on solar wind drivers or geomagnetic indices either before, during, or after the cluster start time. Clusters of substorms linked to moderate (100 nT < AE ≤ 300 nT) or strong AE (AE ≥ 300 nT) disturbances are associated with radiation belt flux enhancements, including up to ultra‐relativistic energies by the strongest substorms (as measured by strong southward Bz and high AE). These clusters reliably occur during times of high speed solar winds streams with associated increased magnetospheric convection. However, substorm clusters associated with quiet AE disturbances (AE ≤ 100 nT) lead to no significant chorus whistler mode intensity enhancements, or increases in energetic, relativistic, or ultra‐relativistic electron flux in the outer radiation belts. In these cases the solar wind speed is low, and the geomagnetic Kp index indicates a lack of magnetospheric convection. Our study clearly indicates that clusters of substorms occurring outside of high speed wind streams are not by themselves sufficient to drive acceleration, which may be due to the lack of pre‐cluster convection.

Solar Cycle Occurrence of Alfvénic Fluctuations and Related Geo‐Efficiency

AbstractWe examine solar wind intervals with Alfvénic fluctuations (ALFs) in 1995–2011. The annual number, the total annual duration, and the average length of ALFs vary over the solar cycle, having a maximum in 2003 and a minimum in 2009. ALFs are most frequent in the declining phase of solar cycle, when the number of high‐speed streams at the Earth's vicinity is increased. There is a rapid transition after the maximum of solar cycle 23 from ALFs being mainly embedded in slow solar wind (<400 km/s) until 2002 to ALFs being dominantly in fast solar wind (>600 km/s) since 2003. Cross helicity increased by 30% from 2002 to 2003 and maximized typically 4–6 h before solar wind speed maximum. Cross helicity remained elevated for several days for highly Alfvénic non‐ICME streams, but only for a few hours for ICMEs. The number of substorms increased by about 40% from 2002 to 2003, and the annual number of substorms closely follows the annual cross helicity. This further emphasizes the role of Alfvénic fluctuations in modulating substorm activity. The predictability of substorm frequency and size would be greatly improved by monitoring solar wind Alfvénic fluctuations in addition to the mean values of the important solar wind parameters.

Effect of prompt penetration on the low latitude ASY indices

Onset of a geomagnetic substorm often intensifies the westward auroral electrojet, as well as produces asymmetric magnetic field at low-/mid-latitudes. Auroral electrojet and low latitude asymmetric indices are known to correlate well during substorms. These indices have been widely used to monitor the duration and strength of substorm activities. However, several processes, other than substorms, introduce local time asymmetry in magnetic field at low latitudes, which can substantially influence the ASY indices. Large number of substorms are observed in association with changes in the interplanetary magnetic field (IMF). It is known that sharp IMF Bz orientation changes result in penetration of interplanetary electric field (IEF) to lower latitudes, which affects the geomagnetic fields to different degree in different local times. In the present study, we demonstrate that sharp IMF Bz fluctuations during the expansion phases of substorms introduce additional asymmetry at low latitudes. The effect is clearly seen in ASYH, whereas ASYD remains almost unaltered.

Substorm and magnetosphere characteristic scales inferred from the SuperMAG auroral electrojet indices

[1] A generalization of the traditional 12-station auroral electrojet (AE) index to include more than 100 magnetometer stations, SME, is an excellent predictor of global auroral power (AP), even at high cadence (1 min). We use this index, and a database of more than 53,000 substorms derived from it, covering 1980–2009, to investigate time and energy scales in the magnetosphere, during substorms and otherwise. We find, contrary to common opinion, that substorms do not have a preferred recurrence rate but instead have two distinct dynamic regimes, each following a power law. The number of substorms recurring after a time Δt, N(Δt), varies as Δt−1.19 for short times ( 3 hours). Other evidence also shows these distinct regimes for the magnetosphere, including a break in the power law spectra for SME at about 3 hours. The time between two consecutive substorms is only weakly correlated (r = 0.18 for isolated and r = 0.06 for recurrent) with the time until the next, suggesting quasiperiodicity is not common. However, substorms do have a preferred size, with the typical peak SME magnitude reaching 400–600 nT, but with a mean of 656 nT, corresponding to a bit less than 40 GW AP. More surprisingly, another characteristic scale exists in the magnetosphere, namely, a peak in the SME distribution around 61 nT, corresponding to about 5 GW precipitating AP. The dominant form of auroral precipitation is diffuse aurora; thus, these values are properties of the magnetotail thermal electron distribution. The characteristic 5 GW value specifically represents a preferred minimum below which the magnetotail rarely drops. The magnetotail experiences continuous loss by precipitation, so the existence of a preferred minimum implies driving that rarely disappears altogether. Finally, the distribution of SME values across all times, in accordance with earlier work on AE, is best fit by the sum of two distributions, each normal in log(SME). The lower distribution (with a 40% weighting) corresponds to the characteristic quiet peak, while the higher value distribution (60% weighting) is an average over the characteristic substorm peak and the subsequent prolonged recovery.

Spring-fall asymmetry of substorm strength, geomagnetic activity and solar wind: Implications for semiannual variation and solar hemispheric asymmetry

Received 12 January 2011; revised 14 February 2011; accepted 18 February 2011; published 24 March 2011. [1] We study the seasonal variation of substorms, geomagnetic activity and their solar wind drivers in 1993–2008. The number of substorms and substorm mean duration depict an annual variation with maxima in Winter and Summer, respectively, reflecting the annual change of the local ionosphere. In contradiction, substorm mean amplitude, substorm total efficiency and global geomagnetic activity show a dominant annual variation, with equinoctial maxima alternating between Spring in solar cycle 22 and Fall in cycle 23. Thelargestannualvariationswerefoundin1994and2003,in the declining phase of the two cycles when high‐speed streams dominate the solar wind. A similar, large annual variation is found in the solar wind driver of substorms and geomagnetic activity, which implies that the annual variation of substorm strength, substorm efficiency and geomagnetic activity is not due to ionospheric conditions but to a hemispherically asymmetric distribution of solar wind which varies from one cycle to another. Our results imply that the overall semiannual variation in global geomagnetic activity has been seriously overestimated, and is largely an artifact of the dominant annual variation with maxima alternating between Spring and Fall. The results also suggest an intimate connection between the asymmetry of solar magnetic fields and some of the largest geomagnetic disturbances, offering interesting new pathways for forecasting disturbances with a longer lead time to the future. Citation: Mursula, K., E. Tanskanen, and J. J. Love (2011), Spring‐fall asymmetry of substorm strength, geomagnetic activity and solar wind: Implications for semiannual variation and solar hemispheric asymmetry, Geophys. Res. Lett., 38, L06104, doi:10.1029/2011GL046751.

Comment on “The importance of auroral features in the search for substorm onset process” by Syun-Ichi Akasofu, A. T. Y. Lui, and C.-I. Meng

Comment on “The importance of auroral features in the search for substorm onset process” by Syun-Ichi Akasofu, A. T. Y. Lui, and C.-I. Meng

Steady magnetospheric convection and stream interfaces: Relationship over a solar cycle

[1] Recently, an association between steady magnetospheric convection (SMC) events and solar wind stream interfaces (SI) was discovered. SMC occurrence tends to peak 0.5–1 day after a stream interface encounters the Earth, while a minimum in SMC events is seen 3–1.5 days before an SI arrives. We investigate whether this relationship holds throughout an entire solar cycle and find that during the declining phase, the relationship is consistent with previous results, while during the rising phase, no association is seen. Conditions during SMCs associated with stream interfaces are compared to unassociated SMCs, and we find differences from previous observations but consistencies with poststream interface conditions. We find that the Russell-McPherron geoeffectiveness of the solar wind stream and increased solar wind–magnetosphere coupling in the declining phase contribute to the observed association. Since SMCs usually begin with a substorm, and substorm occurrence increases both after a stream interface and during the declining phase of the solar cycle, we suggest that the increased number of substorms contributes to the higher probability of an SMC occurring during this period. The question of why coupling is greater, and why substorms occur more frequently during the declining phase, remains to be answered.

북쪽방향 IMF 조건하에서 발생하는 서브스톰의 통계적 특성

While substorms are known to generally occur under southward IMF conditions, they can sometimes occur even under northward IMF conditions. In this paper, we studied the substorms that occurred in May, 2000 to 2002 to examine some statistical characteristics of the IMF and solar wind associated with northward IMF substorms. We focused on the cases where two or more substorms occurred successively under northward IMF conditions. Also, by checking Sym-H index associated with each of the substorms we determined whether or not there is any association of such northward IMF substorm occurrence with storm times. We also examined statistical properties at geosynchronous altitude in terms of magnetic field dipolarization and energetic particle injection. The following results were obtained. (i) Most of the northward IMF substorms occurred under average solar wind conditions. The majority of them occurred within 2 hrs duration of northward IMF Bz state, but there are also a nonnegligible number of substorms that occurred after a longer duraiton of northward IMF Bz state. (ii) While most of the substorms occurred as isolated from a magnetic storm time, those that occurred in a magnetic storm time show a higher average value of IMF and solar wind than that for the isolated substorms. (iii) About 55% of the substorms were associated with the IMF clock angle that can possibly allow dayside reconnection, and the other 45% were associated with more or less pure northward IMF conditions. Therefore, for the latter cases, the energy input from the solar wind into the magnetosphere should be made by other way than the dayside reconnection. (iv) For most of the substorms, the magnetic field dipolarizations and energetic particle injections at geosynchronous altitude were identified to be generally weak. But, several events indicated strong magnetic field dipolarizations and energetic particle injections.

Effect of multiple substorms on the buildup of the ring current

The effect of magnetospheric substorms on the ring current is not completely understood. Using a combination of the University of Michigan's BAT‐S‐RUS Model and Fok Ring Current Model, we modeled the effects of multiple substorms on the ring current by modeling multiple dipolarizations in the tail. Increasing the number of substorms corresponds to increases in the number of injections into the ring current. The ionospheric potential increases during periods of southward IMF. Energy increases are more dependent on the duration of large ionospheric potential than the number of substorm dipolarizations.

Changes of the size of the open field line region during substo rms

The dimensions of both the near noon-midnight and near dawn-dusk meridians of the polar cap are examined during a number of substorms on the basis of particle data taken from the DMSP satellites. There is an annular belt of precipitation of soft electrons (∼300 eV) which extends poleward from the auroral oval. In general, the energy flux of these precipitating soft electrons is rather low and does not always cause sufficient luminosity to be detected by the present satellite-borne aurora imagers. This makes it very difficult to determine, conclusively, the open field line region on the basis of the present auroral imagers. In the present study, the open field line region is determined accurately in terms of the area of void of the soft electron precipitation. It is in this area where the so-called “polar rain” is observed. Our results show clearly that, at least for both medium and weak substorms, the dimension of the open field line region varies roughly in harmony with the AE index. The open region reaches the largest dimension at about the maximum epoch of substorms and becomes smallest during a quiet period, after slowly contracting during the recovery phase. The expansion and contraction of the open region result primarily from a decrease and increase, respectively, of the width of the soft electron precipitation belt, although the hard electron precipitation belt also expands and contracts.

The magnetogram inversion technique: Applications to the problem of magnetospheric substorms

The magnetogram inversion technique (MIT) is based upon recordings of geomagnetic variations at the worldwide network of ground-based magnetometers. MIT ensures a calculation of a global spatial distribution of the electric field, currents and Joule heating in the ionosphere. Variant MIT-2 provides, additionally, continuous monitoring of the following parameters: Poynting vector flux from the solar wind into the magnetosphere (ɛ′); power, both dissipated and accumulated in the magnetosphere; magnetic flux in the open tail; and the magnetotail length (lT) (distance between the dayside and nightside neutral points in the Dungey model). Using MIT-2 and data of direct measurements in the solar wind, an analysis is made of a number of substorms, and a new scenario of substorms is suggested. The scenario includes the convection model, the model with a neutral line and the model of magnetosphere-ionosphere coupling (outside the current sheet), i.e., the three known models. A brief review is given of these and some other substorms models. A new element in the scenario is the strong positive feedback in the primary generator circuit, which ensures growth of the ratio κ = ɛ′/ɛAby an order of magnitude or more during the substorms. Here ɛAis the Pointing vector flux in the Akasofu-Perrault approximation, i.e., without the feedback taken into account. The growth of κ during the substorm is caused only by the feedback effect. It is assumed that the feedback arises due to an elongation of the magnetotail, i.e., a growth of lTby a factor of (2∶3) during the substorm.

Large‐scale characteristics of field‐aligned currents associated with substorms

Characteristics of field‐aligned currents have been determined during a large number of substorms from the magnetic field observations acquired with the Triad satellite. The statistical features of field‐aligned currents include the following: (1) The large‐scale regions of field‐aligned currents determined previously by the authors (Iijima and Potemra, 1976a) persist during all phases of substorm activity, namely, region 1, located near the poleward boundary of the field‐aligned current region, and region 2, located near the equatorward boundary. Field‐aligned currents flow into region 1 on the morningside and away from region 1 on the eveningside. The current flow in region 2 is reversed to region 1 at any given local time except in the Harang discontinuity region (∼2000–2400 MLT), where the flow patterns are more complicated. (2) During active periods (|AL| ≥ 100 γ) the average latitude width of regions 1 and 2 increases by 20–30%, and the centers of these regions shift equatorward by 2°–3° with respect to the quiet time values. (3) The current density in region 1 is statistically larger than the current density in region 2 at all local times except during active periods and in the midnight to morning local time sector. In this region, where the westward electrojet is most active, the current density in region 2 can exceed the current density in region 1. (4) During relatively quiet conditions (|AL| < 100 γ) the largest field‐aligned current densities occur in two areas of region 1 near noon (near ∼ 1030 MLT and ∼ 1300 MLT) with an average value of ∼1.6µA/m². During active periods (|AL| ≥ 100 γ) the regions of peak current density shift toward the nightside (the region near 1030 MLT shifts to ∼0730 MLT, and the region near ∼1300 MLT shifts to ∼1430 MLT), and the average current density increases to ∼2.2 µA/m². (5) The average total amount of field‐aligned current flowing into the ionosphere always equals the current flow away from the ionosphere during a wide range of quiet and disturbed conditions. The average total current during quiet periods is ∼2.7 × 106 A and during disturbed periods is ∼5.2 × 106 A. (6) A three‐region pattern of field‐aligned current flow persists in the Harang discontinuity region (∼2000–2400 MLT) during undisturbed and disturbed periods, when the westward auroral electrojet does not intrude into this sector. This flow pattern consists of an upward flowing field‐aligned current surrounded to the north and south by downward flowing currents. During periods when the westward auroral electrojet has intruded deeply into the evening sector the Triad magnetometer data exhibit complicated and fine‐structured variations indicating the presence of complex field‐aligned currents in this sector. (7) The alignment of current sheets is generally along the boundary of the auroral oval (rather than in the east‐west direction), but noticeable distortions of this alignment occur during very disturbed periods. The alignment of field‐aligned currents is different in region 1 and region 2 during active periods. The different behavior of field‐aligned currents in region 1 and 2 during substorms actively suggests that they are controlled by different source regions in the magnetosphere or ionosphere. The region 1 field‐aligned currents map to the outermost part of the magnetosphere and magnetotail region, whereas the region 2 currents map to regions of the plasma sheet closer to the earth.

Pi 2 pulsations and the auroral electrojet

Measurements of the properties of Pi 2 pulsations along a magnetic meridian at high latitudes during a number of substorms have been analyzed for their relationship to the auroral electrojet. It is found that the maximum Pi 2 pulsation amplitudes are closely associated with the instantaneous position of the electrojet. That is, the average pulsation amplitude in the Pi 2 band as well as the amplitudes of pulsations at specific frequencies in the band have maximum amplitudes at latitudes close to the instantaneous electrojet location. Stations equatorward of the electrojet tend to observe a classical Pi 2 waveform concurrent with the onset of the substorm electrojet. Stations near the electrojet observe a broad spectrum of pulsations indicating a multiplicity of sources. Stations poleward of the initial electrojet position see little pulsation activity until the electrojet moves overhead. The appearance of large amplitude Pi 2 pulsations at a station which was poleward of the electrojet at the onset of a substorm appears to be coincident with the arrival of the poleward border of the electrojet.

Magnetospheric substorms on September 14, 1968

Several moderate magnetospheric substorms on September 14, 1968, were observed by two satellites (Vela 4A and ATS 1), balloon-borne X-ray detectors, and extensive arrays of ground-based instruments in Alaska. Combined results from those observations and their interpretations are described in detail. Several interesting aspects of magnetospheric substorms are suggested by the analysis. (a) Simultaneous with the brightening of an auroral arc that signals the onset of an auroral substorm, there can occur a sudden enhancement of energetic electron flux in a limited region of the distant magnetotail. The simultaneity of these two phenomena suggests a magnetic link between them. The region evidently contracts rapidly toward the earth. A satellite appropriately situated in the magnetotail (e.g., a Vela satellite at 18 RE) may be imbedded in the electron flux initially and then, after a short time, find itself outside it as the flux-containing region contracts earthward. This results in a brief impulsive flux of energetic electrons at the satellite starting at substorm onset. (b) The sharp enhancement of electron flux that, in the midnight sector of the geosynchronous orbit (r=6.6 RE), characterizes substorm onset and that is generally ascribed to a change of electron drift paths associated with earthward ‘collapse’ of magnetic field may, in some substorms, be partly due to earthward motion of a nonadiabatic source region. (c) The westward drift motion of auroral patches results from that of a source or scattering region rather than from the drift of the individual precipitated electrons in a strong electric field. The number of substorms that can be studied with such a variety of simultaneous observations is obviously very limited, and thus the generalization of our conclusions must be taken to be tentative.