Electron loss rates from the outer radiation belt caused by the filling of the outer plasmasphere: The calm before the storm

Measurements from seven spacecraft in geosynchronous orbit are analyzed to determine the decay rate of the number density of the outer electron radiation belt prior to the onset of high-speed-stream-driven geomagnetic storms. Superposed-data analysis is used with a collection of 124 storms. When there is a calm before the storm, the electron number density decays exponentially before the storm with a 3.4-day e-folding time: beginning about 4 days before storm onset, the density decreases from ∼4 × 10−4 cm−3 to ∼1 × 10−4 cm−3. When there is not a calm before the storm, the number density decay is very small. The decay in the number density of radiation belt electrons is believed to be caused by pitch angle scattering of electrons into the atmospheric loss cone as the outer plasmasphere fills during the calms. This is confirmed by separately measuring the density decay rate for times when the outer plasmasphere is present or absent. While the radiation belt electron density decreases, the temperature of the electron radiation belt holds approximately constant, indicating that the electron precipitation occurs equally at all energies. Along with the number density decay, the pressure of the outer electron radiation belt decays, and the specific entropy increases. From the measured decay rates, the electron flux to the atmosphere is calculated, and that flux is 3 orders of magnitude less than thermal fluxes in the magnetosphere, indicating that the radiation belt pitch angle scattering is 3 orders weaker than strong diffusion. Energy fluxes into the atmosphere are calculated and found to be insufficient to produce visible airglow

Magnetic pumping of particles in the outer Jovian magnetosphere

The mechanism of magnetic pumping consists of two processes, the adiabatic motion of charged particles in a time varying magnetic field and their pitch-angle diffusion. The result is a systematic increase in the energy of charged particles trapped in mirror (and particularly, magnetospheric) magnetic fields. A numerical model of the mechanism is constructed, compared with analytic theory where possible, and, through elementary exercises, is used to predict the consequences of the process for cases that are not tractable by analytical means. For energy dependent pitch angle diffusion rates, characteristic 'two temperature' distributions are produced. Application of the model to the outer Jovian magnetosphere shows that beyond 20 Jupiter radii in the outer magnetosphere, particles may be magnetically pumped to energies of the order of 1 - 2 MeV. Two temperature distribution functions with "break points" at 1 - 4 KeV for electrons and 8 - 35 KeV for ions are predicted

Exploring the effect of current sheet thickness on the high‐frequency Fourier spectrum breakpoint of the solar wind

The magnetic power spectrum of the solar wind at 1 AU exhibits a breakpoint at a frequency of about 0.1–1 Hz, with the spectrum being steeper above the breakpoint than below the breakpoint. Because magnetic discontinuities contain much of the Fourier power in the solar wind, it is suspected that current sheet thicknesses (i.e., discontinuity thicknesses) may play a role in determining the frequency of this breakpoint. Using time series measurements of the solar wind magnetic field from the Wind spacecraft, the effect of current sheet thicknesses on the breakpoint is investigated by time stretching the solar wind time series at the locations of current sheets, effectively thickening the current sheets in the time series. This localized time stretching significantly affects the magnetic power spectral density of the solar wind in the vicinity of the high‐frequency breakpoint: a substantial fraction of the Fourier power at the breakpoint frequency is contained in current sheets that occupy a small fraction of the spatial volume of the solar wind. It is concluded that current sheet thickness appears to play a role in determining the frequency fB of the high‐frequency breakpoint of the magnetic power spectrum of the solar wind. This analysis of solar wind data is aided by comparisons with power spectra generated from artificial time series.Key PointsCurrent‐sheet thicknesses affect the high‐frequency breakpoint frequency of the solar windSolar‐wind current sheets contain substantial magnetic Fourier powerThere are outstanding questions about the solar‐wind current sheet origins and physicsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144299/1/jgra52192_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144299/2/jgra52192.pd

The response of the inner magnetosphere to the trailing edges of high‐speed solar‐wind streams

The effects of the leading edge stream interface of high‐speed solar‐wind streams (HSSs) upon the Earth’s magnetosphere have been extensively documented. The arrival of HSSs leads to significant changes in the plasmasphere, plasma sheet, ring current, and radiation belts, during the evolution from slow solar wind to persistent fast solar wind. Studies have also documented effects in the lower ionosphere and the neutral atmosphere. However, only cursory attention has been paid to the trailing‐edge stream interface during the transition back from fast solar wind to slow solar wind. Here we report on the statistical changes that occur in the plasmasphere, plasma sheet, ring current, and electron radiation belt during the passage of the trailing‐edge stream interface of HSSs, when the magnetosphere is in most respects in an extremely quiescent state. Counterintuitively, the peak flux of ~1 MeV electrons is observed to occur at this interface. In contrast, other regions of the magnetosphere demonstrate extremely quiet conditions. As with the leading‐edge stream interface, the occurrence of the trailing‐edge stream interface has a periodicity of 27 days, and hence, understanding the changes that occur in the magnetosphere during the passage of trailing edges of HSSs can lead to improved forecasting and predictability of the magnetosphere as a system.Key PointsThe electron radiation belt flux peaks during the passage of HSS trailing‐edge stream interfaceCounterintuitively, the peak flux occurs when the magnetosphere is in its most calm configurationThe hazard from so‐called killer electrons is maximized; at the same time, hazard from spacecraft surface charging is minimizedPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136314/1/jgra53192.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136314/2/jgra53192_am.pd

A system science methodology develops a new composite highly predictable index of magnetospheric activity for the community: the whole-Earth index E(1)

For community use, a new composite whole-Earth index E(1) and its matching composite solar wind driving function S(1) are derived. A system science methodology is used based on a time-dependent magnetospheric state vector and a solar wind state vector, with canonical correlation analysis (CCA) used to reduce the two state vectors to the two time-dependent scalars E(1)(t) and S(1)(t). The whole-Earth index E(1) is based on a diversity of measures via six diverse geomagnetic indices that will be readily available in the future: SML, SMU, Ap60, SYMH, ASYM, and PCC. The CCA-derived composite index has several advantages: 1) the new “canonical” geomagnetic index E(1) will provide a more powerful description of magnetospheric activity, a description of the collective behavior of the magnetosphere–ionosphere system. 2) The new index E(1) is much more accurately predictable from upstream solar wind measurements on Earth. 3) Indications are that the new canonical geomagnetic index E(1) will be accurately predictable even when as-yet-unseen extreme solar wind conditions occur. The composite solar wind driver S(1) can also be used as a universal driver function for individual geomagnetic indices or for magnetospheric particle populations. To familiarize the use of the new index E(1), its behavior is examined in different phases of the solar cycle, in different types of solar wind plasma, during high-speed stream-driven storms, during CME sheath-driven storms, and during superstorms. It is suggested that the definition of storms are the times when E(1) >1

Particle energization in the inner, nonazimuthally symmetric magnetospheres of neutron stars

The energization process of magnetic pumping, a combination of time dependent magnetic mirror fields with pitch-angle scattering, is applied to trapped charged particles drifting in corotating, azimuthally nonsymmetric neutron star magnetospheres. When particle energization is balanced by synchrotron radiation loss, it is found that protons, rather than electrons, reach considerable kinetic energies and radiate, in the X-ray regime, at rates up to the 10 to the 6th power MeV/proton/sec

Preface: Unsolved problems of magnetospheric physics

The Unsolved Problems of Magnetospheric Physics Workshop was held in September 2015 in Scarborough, UK. In contrast to most other meetings, people were specifically asked not to present and discuss their recent results. Rather, they were asked to bring their opinions and thoughts on unsolved problems to the meeting. Short presentations were encouraged after which the audience would debate and discuss definitions of the problems and how they could be overcome. Were new observations required? New missions? Or simply did the community need to work better together to resolve pertinent and outstanding science questions? Around 50% of the meeting schedule was devoted to discussion sessions on these topics.Key PointsMany unsolved problems exist in magnetospheric physicsThe UPMP workshop discussed these problems and suggested possible solutionsFor some problems, the community already have the data and the tools to make rapid progressPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135163/1/jgra53053_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/135163/2/jgra53053.pd

A Kinetic Alfven wave cascade subject to collisionless damping cannot reach electron scales in the solar wind at 1 AU

(Abridged) Turbulence in the solar wind is believed to generate an energy cascade that is supported primarily by Alfv\'en waves or Alfv\'enic fluctuations at MHD scales and by kinetic Alfv\'en waves (KAWs) at kinetic scales $k_\perp \rho_i\gtrsim 1$. Linear Landau damping of KAWs increases with increasing wavenumber and at some point the damping becomes so strong that the energy cascade is completely dissipated. A model of the energy cascade process that includes the effects of linear collisionless damping of KAWs and the associated compounding of this damping throughout the cascade process is used to determine the wavenumber where the energy cascade terminates. It is found that this wavenumber occurs approximately when $|\gamma/\omega|\simeq 0.25$, where $\omega(k)$ and $\gamma(k)$ are, respectively, the real frequency and damping rate of KAWs and the ratio $\gamma/\omega$ is evaluated in the limit as the propagation angle approaches 90 degrees relative to the direction of the mean magnetic field.Comment: Submitted to Ap

Future beam experiments in the magnetosphere with plasma contactors: The electron collection and ion emission routes

Experiments where a high‐voltage electron beam emitted by a spacecraft in the low‐density magnetosphere is used to probe the magnetospheric configuration could greatly enhance our understanding of the near‐Earth environment. Their challenge, however, resides in the fact that the background magnetospheric plasma cannot provide a return current that balances the electron beam current without charging the spacecraft to such high potential that in practice prevents beam emission. In order to overcome this problem, a possible solution is based on the emission of a high‐density contactor plasma by the spacecraft prior to and after the beam. We perform particle‐in‐cell simulations to investigate the conditions under which a high‐voltage electron beam can be emitted from a magnetospheric spacecraft, comparing two possible routes that rely on the high‐density contactor plasma. The first is an “electron collection” route, where the contactor has lower current than the electron beam and is used with the goal of connecting to the background plasma and collecting magnetospheric electrons over a much larger area than that allowed by the spacecraft alone. The second is an “ion emission” route, where the contactor has higher current than the electron beam. Ion emission is then enabled over the large quasi‐spherical area of the contactor cloud, thus overcoming the space charge limits typical of ion beam emission. Our results indicate that the ion emission route offers a pathway for performing beam experiments in the low‐density magnetosphere, while the electron collection route is not viable because the contactor fails to draw a large neutralizing current from the background.Key PointsThe ion emission route is credible for beam experiments in the magnetosphereThe electron collection route is not viableThe background plasma facilitates beam emissionPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111985/1/jgra51700.pd