During magnetic storms, relativistic electrons execute nearly circular orbits about the Earth and traverse a spatially confined zone within the duskside plasmapause where electromagnetic ion cyclotron (EMIC) waves are preferentially excited. We examine the mechanism of electron pitch-angle diffusion by gyroresonant interaction with EMIC waves as a cause of relativistic electron precipitation loss from the outer radiation belt. Detailed calculations are carried out of electron cyclotron resonant pitch-angle diffusion coefficients Dααfor EMIC waves in a multi-ion (H+, He+, O+) plasma. A simple functional form for Dαα is used, based on quasi-linear theory that is valid for parallel-propagating, small-amplitude electromagnetic waves of general spectral density. For typical observed EMIC wave amplitudes (l-10nT), the rates of resonant pitch-angle diffusion are close to the limit of "strong" diffusion, leading to intense electron precipitation. In order for gyroresonance to take place, electrons must possess a minimum kinetic energy Emin which depends on the value of the ratio (electron plasma frequency/ electron gyrofrequency); Emin also depends on the properties of the EMIC wave spectrum and the ion composition. Geophysically interesting scattering, with Emin comparable to 1 MeV, can only occur in regions where (electron plasma frequency/electron gyrofrequency) ≥ 10, which typically occurs within the duskside plasmapause. Under such conditions, electrons with energy ≥ 1 MeV can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm over a time-scale of several hours to a day.

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Relativistic electron pitch-angle scattering by electromagnetic ion cyclotron waves during geomagnetic storms

The evidence for transient variations in the solar wind dynamic pressure, their effect on magnetopause boundary motion, and the signatures they produce within the magnetosphere and at high-latitude auroral ground stations, are examined. A qualitative model for the magnetospheric response to such variations explaining a wide variety of transient magnetopause, magnetospheric and ground signatures is proposed, and its applicability is shown. The model is presented as an alternative explanation for many of the signatures previously interpreted as direct on site evidence of patchy, sporadic, magnetic field merging, namely the flux transfer events (FTEs).

A model for the transient magnetospheric response to sudden solar wind dynamic pressure variations

Continuous ground-based observations of the dayside aurora provide important information, complementary to the in situ measurements from satellites, on plasma transport and electromagnetic coupling between the magnetosheath and the magnetosphere. In this study, observations of the polar cusp/dayside oval aurora from Svalbard and simultaneous observations of the nightside aurora from Poker Flat, Alaska, and the interplanetary magnetic field from satellites are used to identify the ionospheric signatures of plasma transfer from the solar wind to the magnetosphere. The characteristics of motion, spatial scale, time of duration, and repetition frequency of certain day side auroral forms which occur at the time of large-scale oval expansions (interplanetary magnetic field Bz < 0) are observed to be consistent with the expected optical signatures of plasma transfer through the dayside magnetopause boundary layer, associated with flux transfer events. Similarly, more large-scale (time and space) events are tentatively explained by the quasi steady state reconnection process.

Signatures in the dayside aurora of plasma transfer from the magnetosheath

Magnetospheric chorus emissions : a review

Observations obtained by the Freja satellite at altitudes around 1700 km in the high-latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations ...

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA00855

Ion energization mechanisms at 1700 km in the auroral region

Continuous optical observations of cusp/cleft auroral activities within approximately equal to 09-15 MLT and 70-76 deg magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF B(sub z) changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field-aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Delta p/p is greater than or equal to 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of approximately equal to 5-10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the approximately equal to 10-14 MLT sector, during negative IMF B(sub z) and B(sub y) conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 is less than Delta p/p is less than 0.5). Simultaneous photometer observations from Ny Alesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (approximately equal to 10-12 MLT), before moving into the postnoon sector in the case we study here, when IMF B(sub y) is less than 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive B(sub y). Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF B(sub y) polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed that dayside magnetosphere in the noon or near-noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.

Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field Bz and By

Photometric observations from two stations on Svalbard, Norway, have been used to map the location and dynamics of polar cusp auroras. Cases showing the behavior of cusp auroras and the local magnetic field related to changes in the interplanetary magnetic field (IMF) and irregularities in the solar wind plasma are presented. Dynamical phenomena with different time scales are studied. Southward and northward expansions of the midday sector of the auroral oval are discussed in relation to southward and northward turnings of the magnetosheath magnetic field and geomagnetic substorm activity. Examples showing no direct relationship to substorm activity are presented. Intensifications and rapid motions of discrete auroral structures in the cusp region are shown to be associated with local Pi-type magnetic pulsations, each event lasting a few minutes. These small-scale dynamical phenomena are discussed in relation to the concept of impulsive penetration of plasma irregularities across the dayside magnetopause, from the magnetosheath to the polar cusp region of the magnetosphere.

Large- and small-scale dynamics of the polar cusp

Exploration of the polar upper atmosphere.

Modulated Langmuir waves were commonly observed using the HF waveform-capture instruments on the Freja satellite and SCIFER sounding rocket in the terrestrial auroral zone. The modulation frequency of the Langmuir waves during several hundred events was estimated, and found to extend routinely to several tens of kilohertz. Using the linear dispersion relation for oblique Langmuir waves in the ƒpe < ƒce domain, these modulation band widths were found to correspond to interactions between waves at significantly larger angles to each other, and to B0, than typically treated in theoretical analyses of Langmuir wave modulations. A model of the modulation based on scattering via electrostatic whistler/lower hybrid waves is proposed as a viable explanation of the broad modulation bandwidth.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA01499

Modulated Langmuir waves: Observations from Freja and SCIFER

The Sounding of the Cleft Ion Fountain Energization Region (SCIFER) experiment was conducted to investigate the ionospheric origin of the Cleft Ion Fountain (CIF). In the previous decade several high altitude spacecraft studies concluded that the CIF is the principal source of mass for the magnetosphere, especially O+. Yet the ionospheric cleft in the altitude range between 1000 km and 2000 km had not been explored since the ISIS spacecraft experiments in the 1970s. SCIFER was designed to fill that gap with instrumentation that provided continuous spatial/temporal resolution two orders of magnitude better than that achieved by previous orbiting spacecraft.

J. A. Holtet

Papers

Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field Bz and By

Large- and small-scale dynamics of the polar cusp

Exploration of the polar upper atmosphere.

Modulated Langmuir waves: Observations from Freja and SCIFER

The SCIFER Experiment