Weak Substorm Research Articles

On the formation of postnoon auroral bright spots during weak substorm activity

Postnoon auroral bright spots occurring around 15:00 magnetic local time (MLT) are typically caused by monoenergetic electrons accelerated by quasi-static electric field. In this study, we conduct an analysis of two special auroral spots observed by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program (DMSP) spacecraft. The first bright spot, observed on 05 June 2010, appears as a south-north aligned arc, while the second case, observed on 19 August 2005, takes the form of a circular patch. They both occurred at the equatorward edge of and are connected to the discrete aurora in the auroral oval. Particle measurements indicate that these two postnoon spots are primarily produced by precipitating electrons experiencing an acceleration process by quasi-static electric field. The average energy and integrated energy flux for the two cases are approximately 1.4 keV (1.5 keV) and 2.3 × 1012 (1.1 × 1012) eV/(cm2 s sr). It is suggested that the precipitating particles associated with the two cases originate from the boundary plasma sheet located poleward of the central plasma sheet. During the occurrences, the interplanetary magnetic field (IMF) By was nearly zero, while the IMF Bz was slightly negative, with values above −3 nT. Additionally, the geomagnetic AL index decreased to −160 and −200 nT, respectively. This indicates a potential relationship between the postnoon auroral spots and week substorm activities. Surprisingly, significant ion flow velocity was found to be nearly orientated with the postnoon spots. This suggests that the convection transports plenty of plasma along the south-north direction, offering sufficient particles for the formation of the postnoon auroral bright spots.

Statistics of Magnetosonic Waves in the Slot Region Observed by Van Allen Probes

AbstractWe perform a statistical analysis of magnetosonic (MS) waves in the slot region based on Van Allen Probes observations from September 2012 to February 2018. Our results demonstrate that the wave occurrence rate increases with enhanced geomagnetic activity and decreasing magnetic latitude, with the presence of strongest slot region MS waves near the geomagnetic equator within the 08–20 magnetic local time sector. Power spectral densities of slot region MS waves also intensify during geomagnetically active times, with the occurrence of the major wave power (>∼10−5 nT2/Hz) below ∼25fcp (where fcp is the proton gyrofrequency) and the peak wave intensity (∼10−3 nT2/Hz) below ∼5fcp at L > ∼2.6. A remarkable gap in the MS wave frequency spectrum is also revealed at <∼15fcp during weak substorm activities (AE 300 nT).

Substorm Manifestations at Radio Paths of Oblique Ionospheric Sounding in the Arctic

The impact of geomagnetic substorms on radio propagation within the high-frequency (HF) range was analyzed using oblique ionospheric sounding data. These data were obtained from the unique system of radio paths that covers the whole region of the Russian Arctic. The study focused on the effects of two substorms with a twofold difference in intensity. Variations of four radio propagation parameters were studied: the maximum and lowest observed frequencies of the F2 ionospheric layer (F2MOF and F2LOF), and the same frequencies of the sporadic Es layer (EsMOF and EsLOF). Even the weak geomagnetic substorm (AEmax ~ 500 nT) significantly changed the ionosphere state and, consequently, the character of radio propagation at paths. The absorption of signals was more pronounced during a more intense substorm which resulted in loss of information during the transmissions through HF channels. The variations of propagation parameters (ΔMOF and ΔLOF) depend more on the path reflection point location and on the intensity of a substorm. Two substorms with a twofold difference in intensity had similar effects on ΔMOF, except for (a) the more pronounced smoothing of the Main Effect and (b) the higher ΔEsMOF amplitude during the intense substorm. Both issues are explained by more intense particle precipitations during the more intense disturbance.

A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances

This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cassiopeia A, taken overnight on 18–19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10–80 MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR “core” reveals two different velocities in the scintillation pattern: a primary velocity of ~20–40 ms−1 with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of ~110 ms−1 with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller-scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.

Comparison of the Ionosphere During an SMC Initiating Substorm and an Isolated Substorm

AbstractIn order to assess the effects of ionospheric feedback on different modes of energy transport in the magnetosphere, we investigate an isolated substorm and a steady magnetospheric convection (SMC) event with very similar solar wind drivers. The primary focus is on a comparison between the isolated substorm and the substorm that initiates the SMC. Auroral data from Polar UVI LBHl and LBHs, along with assimilative mapping of the ionosphere electrojet potential patterns are used as inputs to the global ionosphere‐thermosphere model to calculate conductances and Joule heating rates. Results from this study show that the conductances both before and during the events play a large role the ability of the magnetosphere to remain in steady driven state. The substorm that initiates the SMC event shows very different signatures in the ionosphere than isolated substorm; these signatures indicate that there is very weak substorm current wedge, or possibly a pseudo‐breakup.

Magnetotail Fast Flow Occurrence Rate and Dawn-Dusk Asymmetry at XGSM∼-60RE.

As a direct result of magnetic reconnection, plasma sheet fast flows act as primary transporter of mass, flux, and energy in the Earth's magnetotail. During the last decades, these flows were mainly studied within X GSM>−60R E, as observations near or beyond lunar orbit were limited. By using 5 years (2011–2015) of ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun) data, we statistically investigate earthward and tailward flows at around 60 R E downtail. A significant fraction of fast flows is directed earthward, comprising 43% (v x>400 km/s) to 56% (v x>100 km/s) of all observed flows. This suggests that near‐Earth and midtail reconnection are equally probable of occurring on either side of the ARTEMIS downtail distance. For fast convective flows (v ⊥ x>400 km/s), this fraction of earthward flows is reduced to about 29%, which is in line with reconnection as source of these flows and a downtail decreasing Alfvén velocity. More than 60% of tailward convective flows occur in the dusk sector (as opposed to the dawn sector), while earthward convective flows are nearly symmetrically distributed between the two sectors for low AL (>−400 nT) and asymmetrically distributed toward the dusk sector for high AL (<−400 nT). This indicates that the dawn‐dusk asymmetry is more pronounced closer to Earth and moves farther downtail during high geomagnetic activity. This is consistent with similar observations pointing to the asymmetric nature of tail reconnection as the origin of the dawn‐dusk asymmetry of flows and other related observables. We infer that near‐Earth reconnection is preferentially located at dusk, whereas midtail reconnection (X >− 60R E) is likely symmetric across the tail during weak substorms and asymmetric toward the dusk sector for strong substorms, as the dawn‐dusk asymmetric nature of reconnection onset in the near‐Earth region progresses downtail.

Multipoint Observations of Energetic Particle Injections and Substorm Activity During a Conjunction Between Magnetospheric Multiscale (MMS) and Van Allen Probes

AbstractThis study examines multipoint observations during a conjunction between Magnetospheric Multiscale (MMS) and Van Allen Probes on 7 April 2016 in which a series of energetic particle injections occurred. With complementary data from Time History of Events and Macroscale Interactions during Substorms, Geotail, and Los Alamos National Laboratory spacecraft in geosynchronous orbit (16 spacecraft in total), we develop new insights on the nature of energetic particle injections associated with substorm activity. Despite this case involving only weak substorm activity (maximum AE &lt;300 nT) during quiet geomagnetic conditions in steady, below‐average solar wind, a complex series of at least six different electron injections was observed throughout the system. Intriguingly, only one corresponding ion injection was clearly observed. All ion and electron injections were observed at &lt;600 keV only. MMS reveals detailed substructure within the largest electron injection. A relationship between injected electrons with energy &lt;60 keV and enhanced whistler mode chorus wave activity is also established from Van Allen Probes and MMS. Drift mapping using a simplified magnetic field model provides estimates of the dispersionless injection boundary locations as a function of universal time, magnetic local time, and L shell. The analysis reveals that at least five electron injections, which were localized in magnetic local time, preceded a larger injection of both electrons and ions across nearly the entire nightside of the magnetosphere near geosynchronous orbit. The larger ion and electron injection did not penetrate to L &lt; 6.6, but several of the smaller electron injections penetrated to L &lt; 6.6. Due to the discrepancy between the number, penetration depth, and complexity of electron versus ion injections, this event presents challenges to the current conceptual models of energetic particle injections.

The effects of magnetospheric processes on relativistic electron dynamics in the Earth's outer radiation belt

AbstractUsing the electron phase space density (PSD) data measured by Van Allen Probe A from January 2013 to April 2015, we investigate the effects of magnetospheric processes on relativistic electron dynamics in the Earth's outer radiation belt during 50 geomagnetic storms. A statistical study shows that the maximum electron PSDs for various μ (μ = 630, 1096, 2290, and 3311 MeV/G) at L*~4.0 after the storm peak have good correlations with storm intensity (cc~0.70). This suggests that the occurrence and magnitude of geomagnetic storms are necessary for relativistic electron enhancements at the inner edge of the outer radiation belt (L* = 4.0). For moderate or weak storm events (SYM‐Hmin &gt; ~−100 nT) with weak substorm activity (AEmax &lt; 800 nT) and strong storm events (SYM‐Hmin ≤ ~−100 nT) with intense substorms (AEmax ≥ 800 nT) during the recovery phase, the maximum electron PSDs for various μ at different L* values (L* = 4.0, 4.5, and 5.0) are well correlated with storm intensity (cc &gt; 0.77). For storm events with intense substorms after the storm peak, relativistic electron enhancements at L* = 4.5 and 5.0 are observed. This shows that intense substorms during the storm recovery phase are crucial to relativistic electron enhancements in the heart of the outer radiation belt. Our statistics study suggests that magnetospheric processes during geomagnetic storms have a significant effect on relativistic electron dynamics.

In situ evidence for interplanetary magnetic field induced tail twisting associated with relative displacement of conjugate auroral features

[1] We provide in situ evidence for a twisted near-Earth tail configuration that is responsible for the time sequence of conjugate auroral features associated with relative interhemispheric displacement during a weak substorm, as reported by Motoba et al. (2010). We analyzed the magnetic field data observed using four Cluster satellites in the vicinity of 11–14 RE central downtail, in close conjunction with the Iceland-Syowa conjugate optical auroral features. Interestingly, we found that the variations in the magnetic field y component (By) at all satellites correlated moderately well with the variations in the time-shifted interplanetary magnetic field (IMF) clock angle (θCA). The correlation coefficients (0.56 ∼ 0.61) between the By field at Cluster and IMF θCA peaked at a time delay of 52 ± 1 min from the dayside magnetopause, probably corresponding to the time scale for the reconfiguration of the IMF θCA related By field in the near-Earth tail. The IMF θCA related By variation at Cluster, regarded as a manifestation of the twisting magnetotail configuration, also roughly coincided with the relative magnetic local time displacement of nightside conjugate auroral forms. These results provide strong evidence that the reconfiguration (twisting) process of the near-Earth tail on a relatively longer time scale controls the nightside conjugate auroral locations in both ionospheres.

Far tail (255RE) fast response to very weak magnetic activity

[1] We present an original study of the dynamical changes measured in the far tail at XGSM ≈ −255 RE, onboard STEREO-B, related to very weak substorm activity (AE < 100 nT). Three weak auroral electrojet perturbations are well correlated with motions of the far tail in which the spacecraft passes from the lobe to the boundary layer or to the magnetosheath. These boundary motions can hardly be related to a plasmoid as a widening of the tail is expected from such a high-pressure structure. Furthermore, for one of the AE enhancements, ground measurement of auroral luminosity and ground magnetic field provided a precise timing of the substorm onset, thus allowing estimation of the propagation speed of the tail disturbance, supposing it is initiated at ∼20 RE in the midtail. The computed velocity, 1800 km/s, much greater than the typical plasmoid propagation speed, implies that the tail disturbance is due to a large-scale wave propagating inside the lobe, initiated inside the inner plasma sheet at substorm onset and linked to current disruption and/or magnetic reconnection.

Varying interplanetary magnetic field By effects on interhemispheric conjugate auroral features during a weak substorm

Interhemispheric conjugate auroral features during a weak substorm interval were investigated using simultaneous all‐sky camera (ASC) measurements at the northern and southern geomagnetic conjugate points at Tjörnes (TJO; 66.2°N, 342.9°E) in Iceland and Syowa Station (SYO; 69.0°S, 39.6°E) in Antarctica. Around postmidnight, just after the substorm onset, the ASC field of view (FOV) at TJO was first filled with dynamic auroral activations; however, its counterpart was not detected over the zenith at SYO at that time. In contrast, in the late stage (about 20 min after the onset) of substorm development we observed spiral‐like auroral arcs with a similar shape drifting eastward across the center of each ASC FOV, although the one at TJO preceded the one at SYO. The time sequence of the interhemispheric conjugate auroral features was well reflected in the geomagnetic field variations at both stations. On the basis of a detailed comparison of both ASC images, we identified that the northern geomagnetic footprint of SYO was displaced poleward of TJO by up to 3.0° or more in the initial stage of substorm development, whereas in the late stage it was displaced eastward by up to ∼1 h relative to TJO and then moved closer to TJO. We emphasize that the dynamic motion of the conjugate points is a consequence of the time‐dependent magnetotail field reconfiguration process, controlled by the varying interplanetary magnetic field By polarity.

Energetic, ∼5–90 keV neutral atom imaging of a weak substorm with STEREO/STE

We present imaging and high resolution energy spectra of energetic ∼5–90 keV neutral atoms (ENA) of a weak geomagnetic substorm (Dst &gt; −8 nT and AE ≲ 200 nT), made by the Suprathermal Electron (STE) instrument on the STEREO B spacecraft. Enhanced ENA emissions were observed coming from around local midnight near the equator with different spatial distribution and/or temporal behavior at ∼5–20keV compared to ∼20–90 keV. By forward modeling using a parameterized ring‐current model, we show that the ENA images imply the parent equatorial protons have pitch‐angle distributions peaked at 90°, an energy spectrum consistent with in situ proton measurements at geosynchronous orbit, and a spatial asymmetry with the maximum flux at midnight for 5–20 keV and at 2240 MLT for 20–90 keV. These are the first ENA measurements at ∼5 to 26 keV from low altitude, and the first for such weak activity.

Physical and model interpretation of HF radio propagation on the St. Petersburg–Longyearbyen (Svalbard) path

HF radio wave observations have been carried out with an oblique ionospheric sounding (OIS) method on the radio path from St. Petersburg to Longyearbyen (Svalbard), and experimental ionograms were obtained for December 2001. These ionograms have been analysed to investigate the impact of the main ionospheric trough (MIT) and magnetic disturbances on the signals on this path. The observations during weakly disturbed ( K р = 2) magnetic conditions on 14–15 December 2001 were compared with predictions from ray-tracing through a numerical model of the ionosphere. The ray-tracing computer program synthesizes the OIS ionograms by means of the “shooting method”. This method calculates trajectories of HF radio waves for different values of elevation angle and transmission frequency. There was a variety of calculated trajectories, from which we choose those which reach the receiver, and the selected paths provide a synthesis of the oblique ionograms. To simulate HF radio wave propagation, we apply a three-dimensional distribution of the electron density calculated with the mathematical model of the high-latitude ionosphere developed in the Polar Geophysical Institute (PGI). These numerical simulations permit us to interpret specific peculiarities of the OIS data such as abnormal propagation modes, increased delays of signals, enhanced MOF (maximum observed frequency) values etc. New results of the study are summarised as follows. (1) An unusual feature of the propagation along the path is the change of propagation mechanism during substorms on entering a path midpoint (or 1-hop reflection point) to the MIT. (2) Even weak substorms, having the distinguished intensities, lead to the appearance of different types of irregularities observed by the CUTLASS radar and therefore to the different propagation modes and F2MOF values. (3) The PGI model of the ionosphere was first used for ray-tracing at high latitudes. The model results are basically in a good qualitative agreement with experimental observations. This model provides the satisfactory agreement between the calculated and experimental F2MOF values while not correctly representing the fine structure of the experimental OIS ionograms at night. An agreement between the calculated and experimental data is better for day and evening hours than at night.

Response of large‐scale ionospheric convection to substorm expansion onsets: A case study

We have studied the response of large‐scale ionospheric convection to substorm expansion onsets on the basis of two weak substorms of 1 May 2001, during which a large part of the dawn cell of the two‐cell ionospheric convection pattern was monitored by the SuperDARN radars. Ionospheric convection began to enhance first in a localized region of the equatorward part of the dawn cell ∼2 minutes before the expansion onsets of both substorms and then enhanced in the entire dawn cell successively. The enhanced convection persisted throughout their expansion phase, possibly even near the footprint of a plasma sheet region without fast flows observed by Geotail. These observations suggest that ionospheric convection begins to enhance just before substorm expansion onset and then enhances in the entire cell, possibly regardless of the presence of fast earthward flows in the corresponding plasma sheet region of the magnetotail. The global enhancement of ionospheric convection is consistent with that of magnetotail convection, which also begins just before onset.

SAID: Plasmaspheric short circuit of substorm injections

We present magnetically‐conjugate Cluster/DMSP observations of subauroral ion drifts (SAID) after the onset of a weak substorm on 18 March 2002 that confirm and expand on the previous Cluster/DMSP results. The outer side of the SAID channel is aligned with the plasmapause and its outer edge marks the dispersionless boundary of the electron precipitation. The SAID's inner edge co‐locates with a sharp decrease in the flux of the injected ions whose minimum energy increases with the electric potential. Downward ionospheric FACs flow within the channel, mainly near its outer side, concentrating near the density maxima. The SAID peak co‐locates either with that in the density or lies on the outer wall of the trough. The overall SAID features are consistent with a short circuiting of the substorm injection front over the plasmasphere and subsequent formation of a turbulent overlap region.

Observations of Pi2 pulsations by the Wallops HF radar in association with substorm expansion

[1] We report the first sub-auroral Pi2 pulsations observations by a SuperDARN type HF radar. The Wallops radar LOS measurements obtained at ionospheric altitudes, ∼56° magnetic latitude, 23 hour magnetic local time, are shown to be highly correlated with ground magnetic field perturbations obtained at Ottawa. The period of the Pi2 pulsations is 118 s and the m-number is ∼2.3. The availability of both ionospheric LOS measurements and ground based magnetic field perturbations enable us to constrain the properties of the wave. A predominantly shear Alfven mode wave is able to explain both the amplitude and phase relations of our observations. Although the solar wind dynamic pressure is fairly stable the IMF Bz is highly variable thereby preventing an unambiguous identification of a solar wind trigger. Rather, magnetotail observations of a clear dipolarization and an intensification of the auroral westward electrojet producing a modest ground magnetic bay indicate that the wave is associated with the onset of a weak substorm.

Variations of the magnetopause position versus the level of geomagnetic activity (according to data of the INTERBALL-1 Satellite for 1995–1997)

The variations in the deviation of the observed position of the magnetosphere boundary from its mean position predicted by the Shue at al., 1997 (Sh97) model [7] are studied as a function of the substorm activity level (the AE-index value) and magnetic storm intensity (the value of the corrected Dst* index). The results obtained make it possible to state that the amplitude of motion of the magnetospheric boundary on the dayside and in the low-latitude tail is small. It is likely that the position of the boundary is either independent of the AE and Dst* indices or this dependence is weak. At the same time, the boundary of the high-latitude tail shifts inward on the average by 1.5RE with an increase of the AE-index in the case of absence of magnetic storms (contraction of the magnetospheric tail). On the contrary, in the presence of magnetic storms, this boundary shifts outward by up to 3RE with an increase of the AE-index (inflation of the magnetospheric tail). It is also shown that the boundary of the high-latitude tail moves outward with an increase of the Dst* index, both at low substorm activity and in periods of high substorm activity. The amplitude of the outward motion of the high-latitude tail of the magnetosphere is by a factor of two higher for moderate magnetic storms with strong substorms than for moderate magnetic storms with weak substorms.

On the relation between solar wind, pseudobreakups, and substorms

A statistical study of pseudobreakups and substorms is performed using Polar UV images from a 3‐month period in winter 1998–1999. Data from the ACE solar wind monitor are examined in order to determine the influence of solar wind parameters on the occurrence of different substorm and pseudobreakup types. The results confirm that the IMF clock angle and the amount of solar wind energy flux control the strength of a substorm. The majority of large substorms appear when the IMF is strongly southward and the solar wind energy flux is high. Most small substorms occur during weakly positive or zero IMF Bz and low solar wind energy flux values. Pseudobreakups are associated with even lower energy fluxes than small substorms and appear typically for weakly positive IMF Bz. These results are in agreement with the scenario that pseudobreakups essentially are very weak substorms. Pseudobreakups appear during quiet times and during the growth phase or the recovery phase of weak or medium strong substorms. Time periods of enhanced geomagnetic activity with recurrent substorms are devoid of pseudobreakups. A detailed analysis of the different pseudobreakup types reveals that quiet time pseudobreakups appear predominantly during northward IMF. At least 20 percent of these appear at the poleward oval boundary. Optically, they do not differ much from very weak substorms. Growth phase pseudobreakups develop typically at the end of a 1 to 2 hour long excursion from northward to weakly southward IMF and are followed by quite weak substorms. A large majority of recovery phase pseudobreakups occur at a strongly polewardly displaced oval boundary at the end of a very active recovery phase. A considerable decrease of the polar cap size during the preceding substorm is connected to a northward turning of the IMF.

Difference in magnetotail variations between intense and weak substorms

Using Geotail plasma and magnetic field data, we have statistically studied the difference in substorm‐associated magnetotail variations between intense and weak substorms. It was found that for intense substorms the plasmoid‐related southward magnetic field and the first decrease in the total pressure occur closer to the Earth. This finding indicates that the magnetic reconnection site at expansion onset is located closer to the Earth in intense substorms, which is consistent with the dependence of the onset latitude in the ionosphere on the substorm intensity. Also, magnetic field lines during the growth phase especially earthward of the magnetic reconnection site are more stretched in intense substorms than in weak substorms. The dipolarization at X ∼ −10 RE is more significant in intense substorms. Furthermore, the Poynting flux in the lobe toward the plasma sheet and the total pressure during the growth and expansion phases are larger at X ∼ −10 RE than at larger distances, and they are larger in intense substorms than in weak substorms. The enhancement of the Poynting flux and the decrease in the total pressure associated with expansion onset are also more pronounced at X ∼ −10 RE in intense substorms. These results suggest that more energy is accumulated and subsequently dissipated in the near‐Earth magnetotail at X ∼ −10 RE during intense substorms.

Diffuse auroral electron scattering by electron cyclotron harmonic and whistler mode waves during an isolated substorm

There are two main theories for the origin of diffuse auroral electron precipitation: precipitation by electrostatic ECH waves and precipitation by whistler mode waves. Here we analyze a case event where whistler mode hiss, chorus, and ECH waves are intensified during a weak substorm injection event to identify the source of particle precipitation. Examination of the particle data shows that there are three sources of free energy: a temperature anisotropy, a loss cone, and a pancake distribution. Instability analysis shows that the temperature anisotropy excites whistler mode hiss whereas both the temperature anisotropy and the pancake distribution contribute to the excitation of chorus. ECH waves are driven unstable by the loss cone. Wave propagation studies show that the path integrated gain of hiss and chorus is almost unaffected by changes in the depth of the loss cone, whereas ECH waves are very sensitive. Analysis of the changes in the resonant energy during propagation shows that the hiss resonates with electrons above a few keV while chorus resonates below a few hundred eV. As a result, neither hiss nor chorus are likely to cause significant electron precipitation from a few hundred eV to a few keV for this event. On the other hand, ECH waves resonate with electrons in the energy range between that for chorus and hiss. ECH waves can scatter electrons with pitch angles of up to 80° into the loss cone. We conclude that ECH waves are responsible for the formation of the pancake distribution and are probably the main component of diffuse auroral precipitation during this event. We suggest that substorm‐injected electrons are responsible for the intensification of hiss and ECH waves and that rapid scattering of electrons by ECH waves forms the pancake distribution which then excites chorus. We also suggest that rapid pitch angle scattering by ECH waves could be responsible for double frequency banded chorus emissions.