Voyager 1 (V1) began measuring precursor energetic ions and electrons from the heliospheric termination shock (TS) in July 2002. During the ensuing 2.5 years, average particle intensities rose as V1 ...penetrated deeper into the energetic particle foreshock of the TS. Throughout 2004, V1 observed even larger, fluctuating intensities of ions from 40 kiloelectron volts (keV) to >/=50 megaelectron volts per nucleon and of electrons from >26 keV to >/=350 keV. On day 350 of 2004 (2004/350), V1 observed an intensity spike of ions and electrons that was followed by a sustained factor of 10 increase at the lowest energies and lesser increases at higher energies, larger than any intensities since V1 was at 15 astronomical units in 1982. The estimated solar wind radial flow speed was positive (outward) at approximately+100 kilometers per second (km s⁻¹) from 2004/352 until 2005/018, when the radial flows became predominantly negative (sunward) and fluctuated between approximately-50 and 0 km s⁻¹ until about 2005/110; they then became more positive, with recent values (2005/179) of approximately+50 km s⁻¹. The energetic proton spectrum averaged over the postshock period is apparently dominated by strongly heated interstellar pickup ions. We interpret these observations as evidence that V1 was crossed by the TS on 2004/351 (during a tracking gap) at 94.0 astronomical units, evidently as the shock was moving radially inward in response to decreasing solar wind ram pressure, and that V1 has remained in the heliosheath until at least mid-2005.
Early observations indicated that the Earth's Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. ...Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep 'slot' region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth's intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.
Structured features on top of nominally smooth distributions of radiation-belt particles at Earth have been previously associated with particle acceleration and transport mechanisms powered ...exclusively by enhanced solar-wind activity. Although planetary rotation is considered to be important for particle acceleration at Jupiter and Saturn, the electric field produced in the inner magnetosphere by Earth's rotation can change the velocity of trapped particles by only about 1-2 kilometres per second, so rotation has been thought inconsequential for radiation-belt electrons with velocities of about 100,000 kilometres per second. Here we report that the distributions of energetic electrons across the entire spatial extent of Earth's inner radiation belt are organized in regular, highly structured and unexpected 'zebra stripes', even when the solar-wind activity is low. Modelling reveals that the patterns are produced by Earth's rotation. Radiation-belt electrons are trapped in Earth's dipole-like magnetic field, where they undergo slow longitudinal drift motion around the planet because of the gradient and curvature of the magnetic field. Earth's rotation induces global diurnal variations of magnetic and electric fields that resonantly interact with electrons whose drift period is close to 24 hours, modifying electron fluxes over a broad energy range into regular patterns composed of multiple stripes extending over the entire span of the inner radiation belt.
The Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on the two Van Allen Probes spacecraft is the magnetosphere ring current instrument that will provide data for answering the three ...over-arching questions for the Van Allen Probes Program: RBSPICE will determine “how space weather creates the storm-time ring current around Earth, how that ring current supplies and supports the creation of the radiation belt populations,” and how the ring current is involved in radiation belt losses. RBSPICE is a time-of-flight versus total energy instrument that measures ions over the energy range from ∼20 keV to ∼1 MeV. RBSPICE will also measure electrons over the energy range ∼25 keV to ∼1 MeV in order to provide instrument background information in the radiation belts. A description of the instrument and its data products are provided in this chapter.
Broad regions on both sides of the solar wind termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the solar wind have been measured by ...the spacecraft Voyager 1 (refs 1-5) and Voyager 2 (refs 3, 6). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3-5). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August-1 September 2007 (refs 7-9). Unlike Voyager 1, Voyager 2 is making plasma measurements. Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the termination shock structure and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident solar wind.
Our investigation of the long‐term ring current proton pressure evolution in Earth's inner magnetosphere based on Van Allen Probes data shows drastically different behavior of the low‐ and high‐ ...energy components of the ring current proton population with respect to the SYM‐H index variation. We found that while the low‐energy component of the protons (<80 keV) is strongly governed by convective timescales and is very well correlated with the absolute value of SYM‐H index, the high‐energy component (>100 keV) varies on much longer timescales and shows either no correlation or anticorrelation with the absolute value of SYM‐H index. Our study also shows that the contributions of the low‐ and high‐ energy protons to the inner magnetosphere energy content are comparable. Thus, our results conclusively demonstrate that proton dynamics, and as a result the energy budget in the inner magnetosphere, do not vary strictly on storm time timescales as those are defined by the SYM‐H index.
Key Points
We are investigating the long‐term ring current dynamics in the inner magnetosphere.
The high‐energy ring current proton dynamics is not captured by the SYM‐H index variations.
The variations of the energy budget inside GEO are not on the same timescales as the SYM‐H index variations.
Energetic particle transport into the inner magnetosphere during geomagnetic storms is responsible for significant plasma pressure enhancement, which is the driver of large‐scale currents that ...control the global electrodynamics within the magnetosphere‐ionosphere system. Therefore, understanding the transport of plasma from the tail deep into the near‐Earth magnetosphere, as well as the energization processes associated with this transport, is essential for a comprehensive knowledge of the near‐Earth space environment. During the main phase of a geomagnetic storm on 17 March 2013 (minimum Dst ~ −137 nT), the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on the Van Allen Probes observed frequent, small‐scale proton injections deep into the inner nightside magnetosphere in the region L ~ 4 – 6. Although isolated injections have been previously reported inside geosynchronous orbit, the large number of small‐scale injections observed in this event suggests that, during geomagnetic storms injections provide a robust mechanism for transporting energetic ions deep into the inner magnetosphere. In order to understand the role that these injections play in the ring current dynamics, we determine the following properties for each injection: (i) associated pressure enhancement, (ii) the time duration of this enhancement, and (iii) the lowest and highest energy channels exhibiting a sharp increase in their intensities. Based on these properties, we estimate the effect of these small‐scale injections on the pressure buildup during the storm. We find that this mode of transport could make a substantial contribution to the total energy gain in the storm time inner magnetosphere.
Key Points
We investigate ion intensities from the RBSPICE instrument for a storm event
We found multiple ion injections inside geosynchronous throughout the main phase
Their contribution to the total energy gain was found to be substantial
The source of O+ in the storm time ring current Kistler, L. M.; Mouikis, C. G.; Spence, H. E. ...
Journal of geophysical research. Space physics,
June 2016, 2016-06-00, 20160601, Letnik:
121, Številka:
6
Journal Article
Recenzirano
Odprti dostop
A stretched and compressed geomagnetic field occurred during the main phase of a geomagnetic storm on 1 June 2013. During the storm the Van Allen Probes spacecraft made measurements of the plasma ...sheet boundary layer and observed large fluxes of O+ ions streaming up the field line from the nightside auroral region. Prior to the storm main phase there was an increase in the hot (>1 keV) and more isotropic O+ ions in the plasma sheet. In the spacecraft inbound pass through the ring current region during the storm main phase, the H+ and O+ ions were significantly enhanced. We show that this enhanced inner magnetosphere ring current population is due to the inward adiabatic convection of the plasma sheet ion population. The energy range of the O+ ion plasma sheet that impacts the ring current most is found to be from ~5 to 60 keV. This is in the energy range of the hot population that increased prior to the start of the storm main phase, and the ion fluxes in this energy range only increase slightly during the extended outflow time interval. Thus, the auroral outflow does not have a significant impact on the ring current during the main phase. The auroral outflow is transported to the inner magnetosphere but does not reach high enough energies to affect the energy density. We conclude that the more energetic O+ that entered the plasma sheet prior to the main phase and that dominates the ring current is likely from the cusp.
Key Points
Auroral outflow during the storm main phase mainly impacts <1 keV plasma sheet population
The >1 keV (hot) more isotropic plasma sheet O+ population increases prior to the main phase
Inward transport of the hot O+ dominates the ring current; this O+ is likely from the cusp
Two wave packets of second harmonic poloidal Pc 4 waves with a wave frequency of ~7 mHz were detected by Van Allen Probe A at a radial distance of ~5.8 RE and magnetic local time of 13 hr near the ...magnetic equator, where plasmaspheric refilling was in progress. Proton butterfly distributions with energy dispersions were also measured at the same time; the proton fluxes at 10–30 keV oscillated with the same frequency as the Pc 4 waves. Using the ion sounding technique, we find that the Pc 4 waves propagated eastward with an azimuthal wave number (m number) of ~220 and ~260 for each wave packet, respectively. Such eastward propagating high‐m (m > 100) waves were seldom reported in previous studies. The condition of drift‐bounce resonance is well satisfied for the estimated m numbers in both events. Proton phase space density was also examined to understand the wave excitation mechanism. We obtained temporal variations of the energy and radial gradient of the proton phase space density and find that temporal intensification of the radial gradient can generate the two wave packets. The cold electron density around the spacecraft apogee was >100 cm−3 in the present events, and hence the eigenfrequency of the Pc 4 waves became lower. This causes the increase of the m number which satisfies the resonance condition of drift‐bounce resonance for 10–30 keV protons and meets the condition for destabilization due to gyrokinetic effect.
Key Points
The first direct observation of drift‐bounce resonance that excites eastward propagating second harmonic poloidal waves with high m number
These waves are excited by intensification of radial gradient of proton phase space density due to substorm injection
Cold electrons also contribute to the wave excitation by increasing the m number to satisfy gyro‐kinetic destabilization condition
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