M. J. Engebretson

The dependence of high-latitude PcS wave power on solar wind velocity and on the phase of high-speed solar wind streams

We have calculated the integrated ULF wave power in the Pc5 band at two stations, Kevo (part of the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer array in Scandinavia, at auroral zone latitudes), and Cape Dorset (part of the Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada, at cusp latitudes), and compared this power against the solar wind velocity for the last six months of 1993, a period characterized by two persistent high-speed solar wind streams. We find for both local noon at Cape Dorset, and for local morning at Kevo, the Pc5 band power (0.002 – 0.010 Hz) integrated over a six-hour period exhibits a clear power-law dependence on the solar wind velocity. At Cape Dorset we found power α Vsw4, with a correlation coefficient r = 0.73, and at Kevo we found power α Vsw6.5, with r = 0.74. Much of the remaining variation in Pc5 power is due to temporal patterns evident at both stations in response to recurrent high speed streams. Power was strongest at the leading edge of each high speed stream and subsequently decreased more quickly than Vsw. Our observations suggest that it is insufficient to make estimates of Pc5-range ULF wave power on the basis of Vsw alone: one must consider other physical factors, either intrinsic to the solar wind or related to its interaction with Earths magnetosphere. The Kelvin-Helmholtz instability is often considered to play a dominant role in this interaction, and the level of instability depends on both velocity and density. By means of a simple simulation using typical density and velocity values during the passage of a high speed stream, we were able to obtain good agreement with the temporal variations we observed. Finally, this study indicates that ground-based pulsation observations can provide reliable proxies of the initial passage of high speed solar wind streams past Earth.

A comparison of ULF fluctuations in the solar wind, magnetosheath, and dayside magnetosphere: 1. Magnetosheath morphology

“Upstream waves,” generated in the solar wind upstream of a quasi-parallel bow shook, are believed to be a major source of the Pc 3-4 pulsation activity observed in the dayside magnetosphere. In an attempt to better understand the means by which “upstream wave” energy is transmitted from the solar wind into the magnetosphere, we compared simultaneous data from ISEE 1 and 2 in the upstream solar wind, AMPTE IRM in the subsolar magnetosheath, and AMPTE CCE in the dayside magnetosphere. Our observations indicate that dayside magnetospheric Pc 3-4 pulsation activity and low IMF cone angles are associated with increased turbulence in the subsolar magnetosheath magnetic field (with large amplitude fluctuations both parallel and transverse to the average field direction), and with increased and highly variable levels of energetic magnetosheath particles. Fourier analysis of the magnetic field fluctuations shows broadband increases in wave power from 0.01 Hz to at least 0.5 Hz, but with peak power at Pc 3-4 frequencies; there is no evidence in our data set of narrow-band magnetic field variations in the magnetosheath at these times. Purely compressional waves, which are at times observed in the subsolar magnetosheath, have a somewhat narrower frequency distribution, but are associated with neither upstream wave activity nor magnetospheric pulsations.

Magnetometer array for cusp and cleft studies observations of the spatial extent of broadband ULF magnetic pulsations at cusp/cleft latitudes

We have used magnetometer data from 10 locations in Arctic Canada and Greenland, covering over 5 hours in magnetic local time at magnetic latitudes from 75° to 79°, to characterize the dayside patterns of enhanced long-period ULF (10- to 600-s period) wave power at cusp/cleft latitudes. We conclude the following: (1) In agreement with earlier single-station studies, we find that the most common wave type is broadband noise (Pi 1-2). Distinct Pc 3-4 activity and more sustained monochromatic Pc 5 activity are most apparent when this broadband noise is weak. (2) Multistation observations also make clear that strong, broadband Pi 1-2 signals are both temporally and spatially structured: Although their amplitude is somewhat larger near local noon and near nominal cusp latitudes, they often occur simultaneously (to within a few minutes) at all stations. They are thus not local signals, and cannot be interpreted as evidence of passage of an auroral region or boundary over an individual magnetic observatory. In particular, we have found no evidence for a distinctive “cusp” signature in broadband ULF waves in this frequency range. (3) The occurrence of strong broadband Pi 1-2 signals at these latitudes appears to be controlled largely by solar wind velocity. We found good correlations between the occurrence of strong Pi 1-2 signals and high solar wind velocity, and we also noted some dependence on the cone angle of the interplanetary magnetic field for moderate to low solar wind velocities. We speculate that there may be an additional dependence on enhanced levels of trapped plasma in regions topologically connected to the very high latitude dayside ionosphere, such as the entry layer, high-latitude dayside field minimum regions, or plasma mantle. Available satellite data on the level of trapped energetic electron fluxes at geosynchronous orbit showed that broadband power levels appeared to correlate with enhanced flux levels on the time scale of days, but not on shorter time scales, suggesting that any such dependence is not directly related to substorm injections.

Van Allen probes, NOAA, GOES, and ground observations of an intense EMIC wave event extending over 12 h in magnetic local time

Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earths outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large-amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p-p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5–20 cm−3. Waves were also observed by ground-based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA-POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30–80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field-aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC-induced loss processes for this event.

Investigation of EMIC wave scattering as the cause for the BARREL 17 January 2013 relativistic electron precipitation event: A quantitative comparison of simulation with observations

Electromagnetic ion cyclotron (EMIC) waves were observed at multiple observatory locations for several hours on 17 January 2013. During the wave activity period, a duskside relativistic electron precipitation (REP) event was observed by one of the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) balloons and was magnetically mapped close to Geostationary Operational Environmental Satellite (GOES) 13. We simulate the relativistic electron pitch angle diffusion caused by gyroresonant interactions with EMIC waves using wave and particle data measured by multiple instruments on board GOES 13 and the Van Allen Probes. We show that the count rate, the energy distribution, and the time variation of the simulated precipitation all agree very well with the balloon observations, suggesting that EMIC wave scattering was likely the cause for the precipitation event. The event reported here is the first balloon REP event with closely conjugate EMIC wave observations, and our study employs the most detailed quantitative analysis on the link of EMIC waves with observed REP to date.

The spatial extent of radial magnetic pulsation events observed in the dayside near synchronous orbit

We have used simultaneous observations from the AMPTE CCE satellite, in an elliptical orbit with apogee at 8.8 RE, and GOES 5 and GOES 6, in a geostationary orbit at 6.6 RE, to investigate the radial and longitudinal extent of magnetic pulsation events with predominantly radial polarization. Twenty-one events were selected by visual inspection of color-coded Fourier spectrograms produced from data from all three satellites during a several month interval in fall 1984 when the apogee of AMPTE CCE was on the dayside; sixteen events were observed at all three satellites. Local time of the observed events ranged from 0900 to 1900 MLT, but the apparent longitudinal extent of the oscillation region varied considerably from event to event, ranging from the minimum resolution of 1.5 hours MLT (the local time separation of GOES 5 and GOES 6) to 8 hours MLT. Plasma wave data from AMPTE CCE indicated the waves occurred in regions of density characteristic of the outer plasmasphere (∼10 cm−3) and quite far outside the L shell region where densities reached 400 cm−3. These events occurred during magnetically quiet times usually after magnetic storms; interplanetary magnetic field data, when available, indicated an either roughly radial or northward orientation during the events. Wave onset often (but not always) occurred within one hour after sharp drops in the AE index to values of 100 or below. There was no apparent correlation of wave onset or amplitude with plasma beta, which ranged from 0.23 to 1.09 during the nine events presented here. Our frequent observation of the simultaneous onset of waves at different local times with considerably different frequencies reinforces the belief that the onset of these pulsations is determined by an instability that covers some longitudinal extent but that the frequencies are determined by local Alfven resonance conditions, not by the bandwidth of an external source. The data suggest that local plasma density increases associated with plasmaspheric refilling are the immediate cause of local instabilities leading to wave onset; the increase in density may alter the field line resonance conditions to allow the free energy of ∼ 100-keV trapped ions to drive waves via the drift-Alfven-ballooning-mirror mode instability.

Observations of coincident EMIC wave activity and duskside energetic electron precipitation on 18–19 January 2013

Electromagnetic ion cyclotron (EMIC) waves have been suggested to be a cause of radiation belt electron loss to the atmosphere. Here simultaneous, magnetically conjugate measurements are presented of EMIC wave activity, measured at geosynchronous orbit and on the ground, and energetic electron precipitation, seen by the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) campaign, on two consecutive days in January 2013. Multiple bursts of precipitation were observed on the duskside of the magnetosphere at the end of 18 January and again late on 19 January, concurrent with particle injections, substorm activity, and enhanced magnetospheric convection. The structure, timing, and spatial extent of the waves are compared to those of the precipitation during both days to determine when and where EMIC waves cause radiation belt electron precipitation. The conjugate measurements presented here provide observational support of the theoretical picture of duskside interaction of EMIC waves and MeV electrons leading to radiation belt loss.

Pi1 magnetic pulsations in space and at high latitudes on the ground

Nightside, high-latitude (68°–80° magnetic) Pi1 waves, measured with a ground array of induction magnetometers, are studied and compared with magnetic field measurements made at synchronous orbit near the meridian of the ground measurements. The objectives of the study are to relate the ground signatures of Pi1B and PiC to the auroral substorm and its manifestation at synchronous orbit in an attempt to understand the origin of the Pi1 waves. Pi1 waves are measured in the equatorial plane by the GOES spacecraft and appear to be initiated by the dipolarization process of the nightside tail magnetic field at the onset of substorms. Across two meridional arrays of ground stations the earliest onset of Pi1B generally occurred at the lowest-latitude station and, in many instances, this burst was superimposed on a ground signature of a sudden onset of the westward electrojet. In one instance, where good coverage of ground optical data was available, this sudden onset Pi1B was time related to the overhead passage of a westward traveling auroral surge. The timing of maximum Pi1B across the array in both longitude and latitude agrees with the westward motion of the local auroral surge and the poleward motion of the aurora after the surge has arrived at a given site, suggesting a local ionospheric source for some of the Pi1 waves. However, across the entire array, extending about 12° in latitude and 20° in longitude, there often was nearly simultaneous Pi1B wave power at all sites which occurred before the maximum signal at a given site (and presumably local aurora), suggesting horizontal ducting of wave power from the onset of PiIB seen earlier at the auroral zone latitude. Prompt turn-on of PiC waves across the array also indicates ducting of these waves. The narrow bandwidth of the PiC waves themselves suggests a resonant cavity source for them which would indicate that some wave power enters the ionosphere from space (consistent with the GOES in situ data) as the trigger wave for this resonant source mechanism. In conclusion, this study finds evidence for local ionospheric currents, magnetospheric waves, and resonant cavity modes as sources for Pi1 ground waves.

Periodic and quasiperiodic ELF/VLF emissions observed by an array of Antarctic stations

This paper describes amplitude modulations in the frequency range 0–500 mHz of ELF/VLF (0.5–4.0 kHz) radio wave power recorded throughout 1993 and 1995 at Halley and South Pole stations, Antarctica, which lie in approximately the same magnetic meridian and at geomagnetic latitudes (Λ) of 61° and 74°, respectively. Data from the intermediate automatic geophysical observatories P2 and P3 (Λ = 70° and 72°, respectively) were also analyzed where available. In agreement with earlier work, spectrograms have revealed the frequent day-time (typically 0700-1700 MLT) occurrence of modulations lying almost entirely within the two period ranges: 10–60 s and 4–6 s. The first range corresponds to quasiperiodic (QP) emissions, while the latter is typical of the two-hop whistler mode echo period in the plasmatrough, and the events are termed periodic emissions (PEs). QP occurrence rates higher than some earlier studies (335 station-days out of 667 examined) may be attributable to the sensitive spectral analysis technique. The type I QPs (i.e., those correlated with geomagnetic pulsations observed at South Pole and/or P2/P3) were consistent with an upstream wave driver, controlled by the IMF cone angle. Type II QPs (uncorrelated with magnetic pulsations) were always accompanied by PEs, suggesting a link between the two, reinforced by a frequently observed steady increase in period in both phenomena, especially during the morning, possibly associated with increasing densities due to upward flow of photoionized plasma from the ionosphere after dawn. Here we propose that type II QPs are driven by field line resonant ULF waves which in turn are generated by field-aligned currents arising from PE induced electron precipitation.

Magnetospheric Field Line Resonances: A Comparative Planetology Approach

Planetary magnetospheres are natural laboratories for many interesting plasma physical processes which are difficult to study under normal laboratory conditions. Among the major processes occurring in space plasmas are the reconnection phenomenon and field line resonances. This paper deals with the second of these processes. A field line resonance is the resonant coupling between an isotropic mode and an anisotropic mode in a magnetized plasma. Field line resonances allow us to understand many features of ultra-low frequency oscillations in the terrestrial magnetosphere, that is resonant mode coupling is the current paradigm to explain geomagnetic pulsations. A brief historical introduction as well as a physical description of the field line resonance is given. Resonant mode coupling is discussed for the terrestrial, Hermean (Mercury), and Kronian (Saturn) magnetospheres, which represent natural laboratories with different conditions such as size of the laboratory, the background plasma density and composition, and the strength of the magnetic field. This comparative approach allows a deeper insight into the critical coupling problem than an isolated study of the terrestrial field line resonance phenomenon. Finally, resonant mode coupling between elastic wave modes in the solid Earth is briefly tackled and compared with the magnetospheric situation.