Robyn Margaret Millan

Acceleration and loss of relativistic electrons during small geomagnetic storms

Abstract Past studies of radiation belt relativistic electrons have favored active storm time periods, while the effects of small geomagnetic storms (D s t > −50 nT) have not been statistically characterized. In this timely study, given the current weak solar cycle, we identify 342 small storms from 1989 through 2000 and quantify the corresponding change in relativistic electron flux at geosynchronous orbit. Surprisingly, small storms can be equally as effective as large storms at enhancing and depleting fluxes. Slight differences exist, as small storms are 10% less likely to result in flux enhancement and 10% more likely to result in flux depletion than large storms. Nevertheless, it is clear that neither acceleration nor loss mechanisms scale with storm drivers as would be expected. Small geomagnetic storms play a significant role in radiation belt relativistic electron dynamics and provide opportunities to gain new insights into the complex balance of acceleration and loss processes.

Precipitation of relativistic electrons by interaction with electromagnetic ion cyclotron waves

On August 20, 1996, balloon-borne X-ray detectors observed an intense X-ray event as part of a French balloon campaign near Kiruna, Sweden, at 1532 UT (1835 magnetic local time), on an L shell of 5.8. The energy spectrum of this event shows the presence of X rays with energies > 1 MeV, which are best accounted for by atmospheric bremsstrahlung from monoenergetic ∼1.7 MeV precipitating electrons. Ultraviolet images from the Polar satellite and energetic particle data from the Los Alamos geosynchronous satellites show the onset of a small magnetospheric substorm 24 min before the start of the relativistic electron precipitation event. Since the balloon was south of the auroral oval and there was no associated increase in relativistic electron flux at geosynchronous altitude, the event is interpreted as the result of selective precipitation of ambient relativistic electrons from the radiation belts. Pitch angle scattering caused by resonance with electromagnetic ion cyclotron mode waves is the most likely mechanism for selective precipitation of MeV electrons. A model is presented in which wave growth is driven by temperature anisotropies in the drifting substorm-injected proton population. The model predicts that this wave growth and resonance with ∼1.7 MeV electrons will occur preferentially in regions of density >10 cm−3, such as inside the duskside plasmapause bulge or detached plasma regions. The model predictions are consistent with the location of the balloon, the observed energies, and the timing with respect to the substorm energetic particle injection.

Observation of relativistic electron precipitation during a rapid decrease of trapped relativistic electron flux

[1] We present the first quantitative comparison of precipitating and geomagnetically trapped electron flux during a relativistic electron depletion event. Intense bremsstrahlung X-ray emission from relativistic electron precipitation was observed on January 19-20, 2000 (21:20-00:45 UT) by the germanium spectrometer on the MAXIS balloon payload (-7.2 to -9.3 E, 74 S corresponding to IGRF L = 4.7, 1920-2240 MLT). A rapid decrease in the geosynchronous >2 MeV electron flux was simultaneously observed at GOES-8 and GOES-10, and between 0.34-3.6 MeV by GPS ns33 at L = 4.7. The observations show that electrons were lost to the atmosphere early in the flux depletion event, during a period of magnetic field stretching in the tail. The observed X-ray spectrum is well modeled by an exponential distribution of precipitating electrons with an e-folding energy of 290 keV and a lower-energy cut-off of 400 keV. The duration of the event implies precipitation extended over at least 3 hours of MLT, assuming a source fixed in local time. Comparison of the precipitation rate with the flux decrease measured at GPS implies that the loss cone flux was only ∼1% of the equatorial flux. However, precipitation is sufficient to account for the rate of flux decrease if it extended over 2-3 hours of local time.

First detection of a terrestrial MeV X-ray burst

We report the first detection of a terrestrial X-ray burst extending up to MeV energies, made by a liquid-nitrogen-cooled germanium detector (∼2 keV FWHM resolution) on a high-altitude balloon at 65.5° magnetic latitude (L=5.7) in the late afternoon (1815 MLT) during low geomagnetic activity. The burst occurred at 1532–1554 UT on August 20, 1996, and consisted of seven peaks of ∼60–90 s duration, spaced by ∼100–200 s, with quasi-periodic (∼10–20 s) modulation of the peak count rates. The very hard X-ray spectrum extends to the instrumental limit of 1.4 MeV, and is consistent with bremsstrahlung emission from monoenergetic, ∼1.7 MeV, precipitating electrons. Since the trapped relativistic electrons showed a steeply falling energy spectrum from 0.6 to 4 MeV (at L=6.6), the precipitation mechanism appears to be highly energy selective. The modulation frequencies suggest scattering of the MeV electrons due to gyro-resonance with Doppler-shifted electromagnetic ion cyclotron waves, but either equatorial proton densities a factor of ∼10² higher than typical for the plasmasphere or significant O+ densities would be required.

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.

Energetic particle precipitation into the middle atmosphere triggered by a coronal mass ejection

Precipitation of relativistic electrons into the atmosphere has been suggested as the primary loss mechanism for radiation belt electrons during large geomagnetic storms. Here we investigate the geographical spread of precipitation as a result of the arrival of a coronal mass ejection (CME) on 21 January 2005. In contrast to previous statistical studies we provide one of the first attempts to describe the geographic and temporal variability of energetic particle precipitation on a global scale using an array of instruments. We combine data from subionospheric VLF radio wave receivers, the high-altitude Miniature Spectrometer (MINIS) balloons, riometers, and pulsation magnetometers during the first hour of the event. There were three distinct types of energetic electron precipitation observed, one globally, one on the dayside, and one on the nightside. The most extensively observed form of precipitation was a large burst starting when the CME arrived at the Earth, where electrons from the outer radiation belt were lost to the atmosphere over a large region of the Earth. On the dayside of the Earth (10–15 MLT) the CME produced a further series of precipitation bursts, while on the nightside dusk sector (∼20 MLT) a continuous precipitation event lasting ∼50 min was observed at 2.5 < L < 3.7 along with Pc 1–2 pulsations observed with a ground-based magnetometer. These observations suggest that the generation of energetic electron precipitation at the inner edge of the outer radiation belt from electromagnetic ion cyclotron (EMIC) wave scattering into the loss cone is the most direct evidence to date connecting EMIC activity and energetic precipitation.

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.

Global-scale coherence modulation of radiation-belt electron loss from plasmaspheric hiss.

Over 40 years ago it was suggested that electron loss in the region of the radiation belts that overlaps with the region of high plasma density called the plasmasphere, within four to five Earth radii, arises largely from interaction with an electromagnetic plasma wave called plasmaspheric hiss. This interaction strongly influences the evolution of the radiation belts during a geomagnetic storm, and over the course of many hours to days helps to return the radiation-belt structure to its ‘quiet’ pre-storm configuration. Observations have shown that the long-term electron-loss rate is consistent with this theory but the temporal and spatial dynamics of the loss process remain to be directly verified. Here we report simultaneous measurements of structured radiation-belt electron losses and the hiss phenomenon that causes the losses. Losses were observed in the form of bremsstrahlung X-rays generated by hiss-scattered electrons colliding with the Earths atmosphere after removal from the radiation belts. Our results show that changes of up to an order of magnitude in the dynamics of electron loss arising from hiss occur on timescales as short as one to twenty minutes, in association with modulations in plasma density and magnetic field. Furthermore, these loss dynamics are coherent with hiss dynamics on spatial scales comparable to the size of the plasmasphere. This nearly global-scale coherence was not predicted and may affect the short-term evolution of the radiation belts during active times.

Jupiter's radio spectrum from 74 MHz up to 8 GHz

Abstract We carried out a brief campaign in September 1998 to determine Jupiter’s radio spectrum at frequencies spanning a range from 74 MHz up to 8 GHz. Eleven different telescopes were used in this effort, each uniquely suited to observe at a particular frequency. We find that Jupiter’s spectrum is basically flat shortwards of 1–2 GHz, and drops off steeply at frequencies greater than 2 GHz. We compared the 1998 spectrum with a spectrum (330 MHz–8 GHz) obtained in June 1994, and report a large difference in spectral shape, being most pronounced at the lowest frequencies. The difference seems to be linear with log(ν), with the largest deviations at the lowest frequencies (ν). We have compared our spectra with calculations of Jupiter’s synchrotron radiation using several published models. The spectral shape is determined by the energy-dependent spatial distribution of the electrons in Jupiter’s magnetic field, which in turn is determined by the detailed diffusion process across L -shells and in pitch angle, as well as energy-dependent particle losses. The spectral shape observed in September 1998 can be matched well if the electron energy spectrum at L = 6 is modeled by a double power law E − a (1+( E / E 0 )) − b , with a = 0.4, b = 3, E 0 = 100 MeV, and a lifetime against local losses τ 0 = 6 × 10 7 s. In June 1994 the observations can be matched equally well with two different sets of parameters: (1) a = 0.6, b = 3, E 0 = 100 MeV, τ 0 = 6 × 10 7 s, or (2) a = 0.4, b = 3, E 0 = 100 MeV, τ 0 = 8.6 × 10 6 s. We attribute the large variation in spectral shape between 1994 and 1998 to pitch angle scattering, coulomb scattering and/or energy degradation by dust in Jupiter’s inner radiation belts.

Global storm time depletion of the outer electron belt

Abstract The outer radiation belt consists of relativistic (>0.5 MeV) electrons trapped on closed trajectories around Earth where the magnetic field is nearly dipolar. During increased geomagnetic activity, electron intensities in the belt can vary by orders of magnitude at different spatial and temporal scales. The main phase of geomagnetic storms often produces deep depletions of electron intensities over broad regions of the outer belt. Previous studies identified three possible processes that can contribute to the main‐phase depletions: adiabatic inflation of electron drift orbits caused by the ring current growth, electron loss into the atmosphere, and electron escape through the magnetopause boundary. In this paper we investigate the relative importance of the adiabatic effect and magnetopause loss to the rapid depletion of the outer belt observed at the Van Allen Probes spacecraft during the main phase of 17 March 2013 storm. The intensities of >1 MeV electrons were depleted by more than an order of magnitude over the entire radial extent of the belt in less than 6 h after the sudden storm commencement. For the analysis we used three‐dimensional test particle simulations of global evolution of the outer belt in the Tsyganenko‐Sitnov (TS07D) magnetic field model with an inductive electric field. Comparison of the simulation results with electron measurements from the Magnetic Electron Ion Spectrometer experiment shows that magnetopause loss accounts for most of the observed depletion at L>5, while at lower L shells the depletion is adiabatic. Both magnetopause loss and the adiabatic effect are controlled by the change in global configuration of the magnetic field due to storm time development of the ring current; a simulation of electron evolution without a ring current produces a much weaker depletion.