Magnetic Impulse Events Research Articles

Interhemispheric Observations of ULF Waves Caused by Foreshock Transients Under Quiet Solar Wind Conditions

AbstractForeshock transient (FT) events are frequently observed phenomena that are generated by discontinuities in the solar wind. These transient events are known to trigger global‐scale magnetic field perturbations (e.g., ULF waves). We report a series of FT events observed by the Magnetospheric Multiscale mission in the upstream bow shock region under quiet solar wind conditions. During the event, ground magnetometers observed significant Pc1 wave activity as well as magnetic impulse events in both hemispheres. Ground Pc1 wave observations show ∼8 min time delay (with some time differences) from each FT event which is observed at the bow shock. We also find that the ground Pc1 waves are observed earlier in the northern hemisphere compared to the southern hemisphere. The observation time difference between the hemispheres implies that the source region of the wave is the off‐equatorial region.

Fine structure of substorm and geomagnetically induced currents

The rapid changes dB/dt of the Earth’s magnetic field due to substorms can excite large geomagnetically-induced currents (GICs) that can have harmful effects on technological systems. This paper presents an analysis of the characteristics of dB/dt enhancements observed in Fennoscandia using data from the IMAGE magnetometer chain along a geomagnetic longitude of 110 o . The abrupt geomagnetic field variations may be associated with the substorm onsets, isolated magnetic impulsive events (MIEs), and quasi-periodic series of MIEs, known as Ps6 pulsations (periods 5-15 min). For a detailed examination of the latitudinal structure of dB/dt enhancements and their association with the auroral oval boundaries we applied the technique of magnetic keograms. This simple technique helps to visualize and characterize the fine structure of substorm, namely the location in time and latitude of dB/dt enhancements. The location of the auroral oval boundaries has been estimated with the OVATION-prime model. Among the different types of impulsive disturbances investigated auroral substorm onset provides the largest GICs (few tens of A). Isolated nightside MIEs are also effective in the excitation of GICs (>10 A). Ps6 pulsations are effective in the excitation of GICs with magnitude about 20 A and even higher. Monochromatic Pc5 pulsations are capable to induce noticeable GICs, up to ~13 A. Large localized disturbances on short timescales are the actual drivers of GICs, and they may be imagined like geomagnetic “strokes” during magnetospheric “thunderstorms”.

High Latitude Magnetic Impulse Events (MIEs): Polarization Peculiarities and Excitation Conditions

Polarization characteristics (polarization type, ellipticity e, tilt angle τ of the polarization ellipse’s major axis) of high-latitude magnetic impulse events (MIEs) observed at the latitude of the dayside polar cusp are studied. It is established that all impulses are elliptically polarized, being right-polarized in 43% of cases (R-type) and left-polarized in 57% of cases (L-type). The right-polarized MIEs on the ground are more pronounced in the azimuthal direction, whereas the left-polarized events are more clearly marked in the meridional direction. The MIEs of both polarization types have the properties of intermittent processes. It is shown that diurnal and seasonal variations in the occurrence frequency and amplitudes of the events depend significantly on the type of their polarization. The R- and L-type impulse events are predominantly observed during the descending and ascending phase of the solar cycle, respectively. Solar wind high-speed streams (HSSs) are more favorable for exciting right-polarized impulses, whereas left-polarized impulse events are more efficiently excited by coronal mass ejection (CME). It is established that R-type impulses emerge in the conditions when the orientation of the interplanetary magnetic field vector is close to the radial direction against the development of moderate magnetospheric substorms whereas the L-type impulses appear when IMF is perpendicular to the Sun–Earth line in the absence of substorms. The behavior of the characteristics of impulse events significantly depends on the value of the IMF Bz-component and on the angle θxB = arccos(Bx/B). It is conjectured that excitation of the two groups of impulses is caused by the IMF structures in the solar wind stream with the characteristic configuration in the ecliptic plane, which determine the polarization type and properties of MIEs.

Multipoint observation of the response of the magnetosphere and ionosphere related to the sudden impulse event on 19 November 2007

The aim of this study is to provide a complete scope of a magnetic sudden impulse (SI) event along its way through interplanetary space and the magnetosphere until its arrival to the ground. In our case, we chose the event of 19th November 2007 because of the availability of enough well-located spacecraft at that moment for our purpose. We have used a 16 spacecraft data set. We calculated the mass flux variation and the change in magnetic field components across the discontinuity. Thus, we identified the solar wind discontinuity as a shock. We also calculated the orientation of the solar wind shock front. Then, we examined the effects of the shock front propagation in detail. With this large data set, we obtained a global view of the travelling wave front and identified the effects of the compressional wave front. Thus, we determined in detail the shock front passing through the different parts of the magnetosphere. We described the compressional effects in the bow shock, the magnetosheath, and the magnetopause and we depicted the propagation inside the inner magnetosphere. Moreover, we used an extensive data set from magnetic observatories on the ground and so we studied the global distribution of the SI waveform. Finally, the comparison of the observational facts with those derived from the theoretical model showed a good consistency. On the basis of the waveforms and polarizations of this SI, we determined the location in latitude where ionospheric currents (ICs) changed their sense. And also, we related polarization at ground to polarization measured by GOES spacecraft.

Geomagnetic response to solar wind dynamic pressure impulse events at high‐latitude conjugate points

It is commonly assumed that geomagnetic activity is symmetrical between interhemispheric conjugate locations. However, in many cases, such an assumption proved to be wrong. Especially in high‐latitude regions where the magnetosphere and the ionosphere are coupled in a more complex and dynamic fashion, asymmetrical features in geomagnetic phenomena are often observed. This paper presents investigations of geomagnetic responses to sudden change in solar wind pressure to examine interhemispheric conjugate behavior of magnetic field variations, which have rarely been made mainly due to the difficulty of facilitating conjugate‐point measurements. In this study, using magnetometer data from three conjugate stations in Greenland and Antarctica, solar wind pressure impulse events (>5 nPa in <16 min) and their geomagnetic responses, typically seen as magnetic impulse events, have been examined. Our results suggest that asymmetry in ground response patterns between the conjugate locations often shows little correlation with interplanetary magnetic field orientation, season, and ionospheric conductivity, indicating that much more complex mechanism might be involved in creating interhemispheric conjugate behavior.

THEMIS observations of a transient event at the magnetopause

[1] This study focuses on Time History of Events and Macroscale Interactions During Substorms (THEMIS) observations of a long-duration transient event in the vicinity of the dayside magnetopause at ∼15:34 UT on 18 July 2008 that was characterized by features typical of a magnetospheric flux transfer event (FTE): a bipolar negative-positive 5–7 nT signature in the Bn component, a positive monopolar variation in the Bl and Bm components, a ∼5–7 nT enhancement in the total magnetic field strength, and a transient density and flow enhancement. The interplanetary magnetic field (IMF) was mostly radial and disturbed during the intervals studied; that is, it was favorable for the repeated formation, disappearance and reformation of the foreshock just upstream from the subsolar bow shock. We show that varying IMF directions and solar wind pressures created significant effects that caused the compressions of the magnetosphere and the bow shock and magnetopause motions and triggered the transient event. Global signatures of magnetic impulse events (MIEs) in ground magnetograms during the period suggest a widespread pressure pulse instead of a localized FTE as the cause of the event in the magnetosphere. The directions of propagation and the flow patterns associated with the event also suggest an interpretation in terms of pressure pulses.

Dayside proton aurora associated with magnetic impulse events: South Pole observations

We present, for the first time, auroral hydrogen emission at 486.1 nm (proton aurora) associated with a magnetic impulse event (MIE) or a traveling convection vortex (TCV). The MIE (or TCV) is a transient phenomenon (∼5–15 min) characterized by a large amplitude (>50 nT) in the magnetic field disturbance occurring at 70°–80° magnetic latitudes (MLATs). Optical observations at the South Pole Station (−74.3° MLAT) showed a clear spot‐like brightening of proton auroral emissions associated with MIEs. These spots of the proton aurora were ∼300–500 km in length and ∼150–200 km in width at an altitude of 150 km and lasted for ∼1–2 minutes. The spot drifted antisunward with a speed of ∼3–5 km/s but did not always drift. Quasi‐stable spots coincided with poleward moving multiple arcs at 630.0 nm. There was no clear one‐to‐one correspondence between the spots and the power of the ground magnetic field in the Pc1 range. The variety of proton auroral emissions associated with MIEs may be a visible manifestation of the variety of processes that take place near the region generating a pair of field‐aligned currents that drives MIEs.

Investigation of the amplitude features of high-latitude magnetic impulse events

The features of the amplitude distributions of magnetic impulse events have been investigated using the observations from a number of high-latitude observatories in the Northern and Southern hemispheres. It has been shown that the tails of the statistical distribution functions of the impulse amplitudes are approximated by a power law of the form f(A) = A−α, where A is the impulse amplitude and α is the exponent. Therefore, the magnetic-impulse generation regime corresponds to the features of the on-off intermittency model. The distribution of the magnetic impulse amplitudes has been analyzed for various geomagnetic latitudes, local times, seasons, solar activity cycle phases, and interplanetary conditions. It has been found that most statistical distributions of magnetic impulse amplitudes have the exponent α larger than 2, which is typical of the chaotic regimes called “strong turbulence.” In some cases, the exponent α is close to 1, which is typical of the regimes generated in a weakly turbulent medium. Qualitative estimates of the plasma wave turbulence level in the high-latitude magnetosphere have been obtained.

Intermittency in wave processes

The paper generalizes results of study of the intermittency pattern in some wave processes of the burst type in the high-latitude magnetosphere of the Earth. Based on the analysis of distributions of amplitudes and interpeak intervals, it is shown that generation regimes of magnetic impulse events, ipclpulsation burst series, and Pi3 pulsations possess the properties of intermittent processes and are associated with the development of plasma turbulence in their generation regions. The degree of turbulence of the high-latitude magnetosphere plasma is estimated at a qualitative level. Burst-type structures whose behavior corresponds to a typical intermittent process are discovered in variations of the ancient geomagnetic field.

THEMIS observations of a hot flow anomaly: Solar wind, magnetosheath, and ground‐based measurements

The THEMIS spacecraft encountered a Hot Flow Anomaly (HFA) on the dusk flank of the Earth’s bow shock on 4 July 2007, observing it on both sides of the shock. Meanwhile, the THEMIS ground magnetometers traced the progress of the associated Magnetic Impulse Event along the dawn flank of the magnetosphere, providing a unique opportunity to study the transmission of the HFA through the shock and the subsequent downstream response. THEMIS‐A, in the solar wind, observed classic HFA signatures. Isotropic electron distributions inside the upstream HFA are attributed to the action of the electron firehose instability. THEMIS‐E, just downstream, observed a much more complex disturbance with the pressure perturbation decoupled from the underlying discontinuity. Simple calculations show that the pressure perturbation would be capable of significantly changing the magnetopause location, which is confirmed by the ground‐based observations.

Patterns of simultaneous observations of high-latitude magnetic impulses (MIEs) and impulsive bursts in the Pc1–2 band

The analysis of simultaneous observations of 128 cases of high-latitude magnetic impulse events (MIEs), as well as geomagnetic pulsations in the Pc1–2 band observed in the area of the dayside cusp, was carried out. We investigated magnetograms from the Mirny Observatory, Antarctica. As a result of the examination, three groups of impulses were identified: (1) impulses accompanied by impulsive bursts of intervals of pulsations with rising periods (IPRPs)-type geomagnetic pulsations—16% of all events, (2) impulses accompanied by impulsive bursts of the Pi1B type (bursts of irregular magnetic pulsations)—48% of all events, and (3) impulses which were not accompanied by geomagnetic pulsations within a high-frequency band—36% of all events. It was found that the maximum frequency of occurrence of the impulses accompanied by impulsive bursts of the IPRP and Pi1B types was observed between 1200 and 1300 MLT. The events of the first two groups were observed predominantly when Bz>0. It was shown that the filling frequency of impulsive bursts that accompany the occurrence of impulses depends on the amplitude of the bursts. The maximum frequency of the occurrence of impulses which were not accompanied by impulsive bursts is between 1000 and 1100 MLT. The events of the third group were observed predominantly when Bz<0. In most cases, the occurrence of impulsive bursts coincided with the leading edge of the MIE. It is supposed that the MIE generation is stimulated by intensification of the plasma turbulence level at the dayside magnetopause in consequence of modulation instability development or reconnection processes.

Interplanetary magnetic field control of dayside transient event occurrence and motion in the ionosphere and magnetosphere

Abstract. The pressure pulse model for dayside transient ionospheric events predicts dawnward moving events at and prior to local noon during periods of spiral interplanetary magnetic field (IMF) orientation, but duskward moving events at and after local noon during rarer periods of orthospiral IMF orientation. We use this model to interpret ground and geosynchronous magnetometer observations of a duskward-moving transient event that occurred on 10 August 1995 during a period of orthospiral IMF orientation. We then survey geosynchronous GOES-8, 9, and 10 magnetometer observations to determine the directions of motion for 67 isolated magnetic impulse events seen in South Pole magnetograms from 1995-1999. The occurrence patterns and directions of motion inferred from both case and statistical studies are consistent with pressure pulse model predictions. Key words. Interplanetary physics (Interplanetary magnetic fields; discontinuities) – Magnetospheric physics (Magnetosphere-ionosphere interactions)

Occurrence statistics of magnetic impulsive events

Abstract. In this study, we perform a statistical investigation of magnetic impulse events identified in the Greenland magnetometer stations through the years 1995–2001. We focus on occurrence statistics that can be determined reliably with an automatic event identification procedure. Durin the first two years we observed almost 40% more events than in the following years. Season is not a significant factor in event occurrence. Event occurrence peaks near 12:00 UT, corresponding to approximately 10:00 magnetic local time (MLT) at the west coast of Greenland. More events occur prior to local noon than after. Event days are not distributed evenly. Large amplitude events, particularly, tend to appear on consecutive days. Events are observed at lower latitudes at earlier local times in a way consistent with the projection of the outer magnetospheric boundary into the ionosphere. Event latitude depends on dipole tilt angle in a manner similar to that reported for the cusp. Events occur during intervals of enhanced Kp. The main reason for this is that the events themselves contribute to the Kp index. Events exhibit a preference for high solar wind velocity. In particular, the large amplitude events occur during high-speed streams. A slight preference for lower density and more radial interplanetary magnetic fields, as compared to the nominal solar wind distribution, is also observed. However, both the nominal solar wind and event distribution exhibit large differences from year to year, indicating that events occur under a broad range of conditions. Key words. Ionosphere (ionospheric disturbances) Magnetospheric physics (magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

Statistical identification of solar wind origins of magnetic impulse events

We have compiled a complete list of magnetic impulse events (MIEs) during the 8‐year period of 1995–2002 covering solar minimum to solar maximum using fluxgate magnetometer data obtained at the South Pole. Wavelet analysis enables us to detect 825 distinct MIEs automatically with high confidence. Interplanetary magnetic field (IMF) discontinuities are also detected automatically for the same period from Wind and ACE satellite data. From an examination and comparison of the two lists, we find that monthly and seasonal variations in IMF discontinuity occurrences have a significant correlation (0.4–0.5) with those of MIEs. We also find that MIEs tend to occur during intervals of low density, high‐speed solar wind streams, and/or radial IMF. No preferences for MIE occurrences are found for solar wind pressure jumps or IMF Bz southward turnings. From detailed minimum variance analysis of 36 IMF discontinuities with one‐to‐one correspondence to MIEs, ∼70% of the IMF discontinuities are found to be tangential discontinuities. The hot flow anomaly (HFA) mechanism can explain at most ∼50% of the MIEs; bursty reconnection and pressure pulses can explain the production of at most ∼30% and ∼20% of the MIEs, respectively. All of the observations and associations are consistent with HFAs or foreshock cavities being the main cause of MIEs.

Multistation studies of the simultaneous occurrence rate of Pc 3 micropulsations and magnetic impulsive events

This study reports simultaneous observations of two different phenomena whose sources may be either near the magnetopause or in the solar wind: Pc 3 micropulsations and high‐latitude magnetic impulsive events (MIEs), whose best studied subset comprises traveling current vortices (TCVs). Isolated Pc 3 emissions that occurred simultaneously with MIE or TCV events and appeared to have the same horizontal speed as the TCVs have been reported. This observation suggests that these events were on the same geomagnetic field lines and may share a common source. The current study reports a search of existing databases for simultaneous Pc 3‐MIE observations. The databases that were searched are the South Pole search coil magnetometer data during 1985–1986 and the Magnetometer Array for Cusp and Cleft Studies (MACCS) database from 1992 to 1993. The rate of occurrence of joint Pc 3 bursts and high‐latitude impulsive events has been consistently found to be ∼70–90%, depending on the database. The rate of coincidence is so high that it suggests that search algorithms based on ground‐based data that seek “clean” MIE signals may be omitting some events owing to the presence of large‐amplitude ULF emissions. A reverse search for candidate MIEs occurring during dayside Pc 3 bursts doubled the number of candidate MIEs in the database. The final results show that 91% of all MIEs were accompanied by Pc 3 emissions, and 43% of all 10–30 min bursts or banded enhancements in Pc 3 emission were accompanied by an MIE. A statistical analysis confirms that the correlation rates are statistically significant. The observed dependence of the properties of the MIE‐concurrent Pc 3s on the solar wind and interplanetary magnetic field is consistent with previous studies of Pc 3s of solar wind origin.

Traveling convection vortices induced by solar wind tangential discontinuities

Two typical magnetic impulse events (MIEs) accompanied by traveling convection vortices (TCVs) are investigated. The analysis of their conjugate equivalent convection patterns is performed using magnetic field data obtained from high‐latitude ground magnetometer networks in the Northern and Southern Hemispheres. A three‐dimensional analysis of solar wind structures is also performed using solar wind data obtained from multiple International Solar‐Terrestrial Physics satellites. In the first event observed at ∼1310 UT on 22 May 1996, a westward moving TCV appeared simultaneously in the noon‐to‐dawn sector in the Northern and Southern Hemispheres. The solar wind source of this TCV is found to be a tangential discontinuity (TD), which causes a rapid northward turning of the interplanetary magnetic field (IMF) and abrupt dynamic pressure changes. In the second event observed at ∼1610 UT on 27 May 1998, an eastward moving TCV appeared in the noon sector in the Northern and Southern Hemispheres, with a timing delay of 2 to 3 min in the Southern Hemisphere. The solar wind source of this TCV is found again to be a TD, which causes a rapid IMF By negative turning and an abrupt enhancement of dynamic pressure. Analyses show that the TDs driving these events have their motional electric fields pointing toward the TDs and their normal vectors with large cone angles from the sunward direction. These TDs satisfy the conditions for the formation of a hot flow anomaly (HFA) at the bow shock. The sweeping motion across the magnetosphere of the intersection of the TD and the bow shock is found to be consistent with the observed TCV motion in each event. Magnetopause deformations due to HFAs can explain all the observed morphological features and the triggering process of these two MIEs. It is suggested, however, that bursty merging and/or pressure pulses would reinforce the processes produced by the HFAs, since the TDs are usually accompanied by both abrupt IMF changes and pressure enhancements. Consequently, it seems reasonable to conclude that the integrated processes of HFA, bursty magnetic field merging, and pressure pulses produce the evolution of these MIEs and TCVs.

Quiet time variability of the geosynchronous magnetic field and its response to the solar wind

We present a survey of the variability of the geosynchronous magnetic field strength on the dayside using observations by the GOES satellites over a period exceeding 4 years. Only intervals of reduced geomagnetic activity, as defined by Dst &gt; −20 nT, were considered in this study. The magnetic field strength data were filtered with a passband of 1.7 mHz to 17 mHz (1–10 minutes), a process that eliminates the diurnal variation of the field strength and the effects of most of the higher frequency (&gt;17 mHz) ultralow‐frequency (ULF) waves. The geosynchronous field strength appears to exhibit the greatest variability in the prenoon sector for spiral interplanetary magnetic fields (IMF) and in the postnoon sector for orthospiral IMF, suggesting that pressure pulses generated in the foreshock/bow shock region may have a significant influence on the geosynchronous field. The seasonal dependence of the variability was determined to be positively correlated to the seasonal dependence of ground‐based observations of magnetic impulse events. The response of the variability of the geosynchronous magnetic field strength around local noon to solar wind parameters was also studied. Here, we observed that the variability was strongly affected by changes in the solar wind dynamic pressure but was seemingly independent of the northward/southward direction of the interplanetary magnetic field. However, for high solar wind dynamic pressures, the variability was found to be greater for northward IMF than for southward IMF.

Magnetic impulse events at the nightside auroral latitudes and associated phenomena

The connection between auroral activation and magnetic impulse events, (MIE) on the nightside is the issue of this study. MIE can be seen during the growth phase and onset of a substorm in association with field-aligned currents (FACs) connecting the auroral ionosphere and plasma sheet. We suggest that the intensity of the FAC studied with magnetometers, pulsation magnetometers, riometer and optical data can be regarded as a key parameter for characteristic of the magnitude of energy released during each activation. The threshold value of the FAC density contributing to a substorm intensification is estimated in the present study.

Polar UVI observations of dayside auroral transient events

We analyze Polar Ultraviolet Imager (UVI) observations of auroral transient events (ATEs) in the dayside Northern Hemisphere. During 5 winter months in 1996 and 1997, we found 31 prenoon ATEs but only 13 afternoon events. Prenoon and afternoon event characteristics differ. Prenoon ATEs generally appear as bright spots of auroral luminosity in the area from 0800 to 1000 magnetic local time (MLT) and 74.5° and 76.5° corrected geomagnetic latitude (CGL). Bright aurorae then quickly expand westward and poleward, accompanied by high‐latitude magnetic impulsive events (MIE) and traveling convection vortices (TCV). Afternoon ATEs usually appear as a sudden intensification of aurorae in the area from 1400 to 1600 MLT and 75.5° to 78.5° CGL. Within 15‐20 min the bright band of luminosity extends eastward to reach 2000–2100 MLT at 70°–72° CGL. Although midlatitude and low‐latitude ground magnetograms in the evening sector record increases in the horizontal component of the magnetic field, no corresponding features occur at stations in the morning sector. Afternoon ATEs correspond to abrupt changes in the interplanetary magnetic field (IMF) orientation, but not to significant variations of the solar wind dynamic pressure, indicating that the auroral transient events occur as part of the magnetospheric response to abrupt changes in the foreshock geometry.

Solar wind effects on ionospheric convection: a review

The solar wind, magnetosphere, and ionosphere are intrinsically coupled through magnetic field lines. The electrodynamic state of the high-latitude ionosphere is controlled by several geophysical processes, such as the location and rate of magnetic reconnection at the magnetopause and in the magnetotail, and the energisation and precipitation of solar wind and magnetospheric plasmas. Amongst the most observed ionospheric manifestation of solar wind/magnetospheric processes are the convection bursts associated with the so-called flux transfer events (FTEs), magnetic impulse events (MIEs), and travelling convection vortices (TCVs). Furthermore, the large-scale ionospheric convection configuration has also demonstrated a strong correspondence to variations in the interplanetary medium and substorm activity. This report briefly discusses the progress made over the past decade in studies of these transient convection phenomena and outlines some unsettled questions as well as future research directions.