Geoff Reeves

Reanalysis of relativistic radiation belt electron fluxes using CRRES satellite data, a radial diffusion model, and a Kalman filter

[1]xa0In this study we perform a reanalysis of the sparse MEA CRRES relativistic electron data using a relatively simple one-dimensional radial diffusion model and a Kalman filtering approach. By combining observations with the model in an optimal way we produce a high time and space resolution reanalysis of the radiation belt electron fluxes over a 50-d period starting on 18 August 1990. The results of the reanalysis clearly show pronounced peaks in the electron phase space density (PSD), which can not be explained by the variations in the outer boundary, and can only be produced by a local acceleration processes. The location of the innovation vector shows that local acceleration is most efficient at L* = 5.5 for electrons at K = 0.11 G0.5REand μ = 700 MeV/G. Sensitivity numerical experiments for various values of μ and K indicate that peaks in PSD become stronger with increasing K and μ. To verify that our results are not affected by the limitations of the satellite orbit and coverage, we performed an “identical twin” experiments with synthetic data specified only at the locations for which CRRES observations are available. Our results indicate that the model with data assimilation can accurately reproduce the underlying structure of the PSD even when data is sparse. The identical twin experiments also indicate that PSD at a particular L-shell is determined by the local processes and cannot be accurately estimated unless local measurements are available.

Rapid enchancements of relativistic electrons deep in the magnetosphere during the May 15, 1997, magnetic storm

Variations in 0.2-3.2 MeV electron flux in the magnetosphere during the May 15, 1997, magnetic storm (the largest magnetic storm of 1997) are examined. After over an order of magnitude initial decrease of the 0.2-3.2 MeV electron fluxes, the 0.2- 0.8 MeV electron flux at L , 4.5 increased and surpassed the prestorm level in an hour. This increase was followed by increases of the more energetic 0.8 -3.2 MeV electron fluxes. These energetic electron variations are examined utilizing data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), the Global Positioning System (GPS) series of satellites, and the Los Alamos National Laboratory (LANL) sensors on board geosynchronous satellites. During the main phase of the storm, fluxes of .0.4-MeV electrons from SAMPEX decreased at L. 4.5 following the Dst drop but increased somewhat at L , 4. GPS satellite data also show that the electron flux decreased in the energy range 0.2-3.2 MeV for all L values above the minimum detectable L value of ;4.2 simultaneously with the decrease in Dst, which is consistent with an adiabatic process. However, the recovery of the electron flux was different at different energies, with an earlier recovery of the less energetic electrons and a later recovery of the more energetic electrons. The recovery of the electron fluxes started before the recovery of Dst, indicating that nonadiabatic processes were involved. The 0.2- to 0.8-MeV electrons appeared in the low-L region (4.2- 4.5) at about the same time that the GOES 9 spacecraft measured a strong dipolarization of the Earths stretched magnetic field. Outer zone electron fluxes continued to increase across a wide L range (L 5 3-8) though the electron flux exhibited a strong spatial gradient, with the peak flux below L 5 4.2 in the equatorial plane. These data are used to test the idea if radial transport from larger L can account for all of the increase in the flux in the heart of the outer zone electron radiation belt at L 5 4-5. However, the radial gradient of the phase space density for a given first adiabatic invariant was estimated to be negative as a function of radial distance during the time that the electron flux was increasing. This estimate is somewhat uncertain because of rapid temporal variations and sparse data. However, if this estimate is correct, the usual theory of radial transport from larger radial distances cannot account for all of the increase in the electron flux. The analysis thus suggests that another process, such as local heating, which does not conserve m, may be required to explain the subsequent enhancement of the more energetic (0.8- to 3.2-MeV) electrons but that additional data are required to answer this question definitely.

On phase space density radial gradients of Earth's outer‐belt electrons prior to sudden solar wind pressure enhancements: Results from distinctive events and a superposed epoch analysis

[1]xa0Here, we present the results of a study of phase space density radial gradients for outer-belt electrons at and beyond geosynchronous orbit prior to 86 sudden solar wind pressure enhancements from 1993 through 2007. All of the events are classified and analyzed based on the results for equatorial electrons with first adiabatic invariants of 50, 200, 750, and 2000 MeV/G. Examples of three distinctive events are compared, and the results from a superposed epoch analysis are presented. We find that the radial gradients are dependent on the first adiabatic invariant (i.e., energy), and that for the majority of cases, the gradient is negative for electrons with energies above a couple of hundred keV, while it is either positive or relatively flat for electrons with energies lower than this, which is evidence of two distinct populations. In the cases where a positive gradient is observed for 2000 MeV/G electrons, the solar wind and geomagnetic conditions are very quiet for at least two days prior to the event, but for the events when the gradient for the same electrons is negative, there is a consistent evidence of enhanced substorm activity and/or convection in the days leading up to the events. Overall, this study puts previous observations of phase space density (PSD) gradients into a broader context of solar wind and geomagnetic conditions, while encompassing a broad range of energies, from the source population of tens to hundreds of keV electrons to relativistic electrons with energies exceeding 1 MeV. We discuss how 41 of the 86 events are consistent with and can be explained by local heating by wave-particle interactions, and we provide evidence of the solar wind and geomagnetic conditions that are important to different types of sources of outer-belt electron PSD.

Highly relativistic radiation belt electron acceleration, transport, and loss: Large solar storm events of March and June 2015

Abstract Two of the largest geomagnetic storms of the last decade were witnessed in 2015. On 17 March 2015, a coronal mass ejection‐driven event occurred with a Dst (storm time ring current index) value reaching −223u2009nT. On 22 June 2015 another strong storm (Dst reaching −204u2009nT) was recorded. These two storms each produced almost total loss of radiation belt high‐energy (Eu2009≳u20091u2009MeV) electron fluxes. Following the dropouts of radiation belt fluxes there were complex and rather remarkable recoveries of the electrons extending up to nearly 10u2009MeV in kinetic energy. The energized outer zone electrons showed a rich variety of pitch angle features including strong “butterfly” distributions with deep minima in flux at αu2009=u200990°. However, despite strong driving of outer zone earthward radial diffusion in these storms, the previously reported “impenetrable barrier” at Lu2009≈u20092.8 was pushed inward, but not significantly breached, and no Eu2009≳u20092.0u2009MeV electrons were seen to pass through the radiation belt slot region to reach the inner Van Allen zone. Overall, these intense storms show a wealth of novel features of acceleration, transport, and loss that are demonstrated in the present detailed analysis.

Electric field structures and waves at plasma boundaries in the inner magnetosphere

Recent observations by the Van Allen Probes spacecraft have demonstrated that a variety of electric field structures and nonlinear waves frequently occur in the inner terrestrial magnetosphere, including phase space holes, kinetic field line resonances, nonlinear whistler mode waves, and several types of double layer. However, it is unclear whether such structures and waves have a significant impact on the dynamics of the inner magnetosphere, including the radiation belts and ring current. To make progress toward quantifying their importance, this study statistically evaluates the correlation of such structures and waves with plasma boundaries. A strong correlation is found. These statistical results, combined with observations of electric field activity at propagating plasma boundaries, are consistent with the scenario that the sources of the free energy for the structures and waves of interest are localized near and comove with these boundaries. Therefore, the ability of these structures and waves to influence plasma in the inner magnetosphere is governed in part by the spatial extent and dynamics of macroscopic plasma boundaries in that region.

Are sawtooth oscillations of energetic plasma particle fluxes caused by periodic substorms or driven by solar wind pressure enhancements

[1]xa0Energetic electron and proton fluxes measured by geosynchronous satellites often show sawtooth-like variations during magnetic storms. We examine whether the sawtooth oscillations and relevant magnetospheric-ionospheric disturbances are caused by periodic substorms or driven by a series of enhancements in the solar wind pressure. We show that there are significant differences between periodic substorms and solar wind-induced variations. The energetic fluxes at geosynchronous orbit may increase by orders of magnitude after each onset of periodic substorms and by 10–50% in response to a large solar wind pressure impulse. The sudden increases of the energetic fluxes during periodic substorms show significant time delays of 30–50 min at different longitudes/local times, indicating that the fluxes are injected on the nightside and then drift to the dayside. In contrast, the small flux increases caused by solar wind pressure enhancements occur almost simultaneously at all local times. The periodic substorms always have a strong spectrum peak at 2–3 hours, no matter whether the solar wind pressure and/or IMF have similar spectrum peaks. The nightside magnetospheric magnetic elevation angle shows a large (30–60°) increase at each onset of periodic substorms through dipolarization and a small (<10°) decrease in response to a solar wind pressure impulse. Each cycle of periodic substorms can cause a deviation of 40–60 nT in the midlatitude geomagnetic field; the midlatitude geomagnetic deviations caused by solar wind pressure enhancements are proportional to the square root of the pressure change. The increase of the polar cap index caused by substorms is ∼4 times that caused by solar wind pressure enhancements. We conclude that the sawtooth-like flux oscillations represent flux injections during periodic substorms and that the period of substorms is determined by the magnetosphere.

Periodic magnetospheric substorms during fluctuating interplanetary magnetic field Bz

[1]xa0We study the possible role played by sudden changes in the solar wind in triggering substorm onsets. We show that periodic substorms can occur when the IMF Bz is small and fluctuating between southward and northward. The period of the substorms during the fluctuating IMF is ∼3 hours, nearly the same as that during strongly and continuously southward IMF. We also show that large sudden changes in the solar wind do not necessarily trigger substorm onsets. For example, a northward IMF turning with a change of 31 nT in the IMF Bz after 1.2 hours of strongly southward IMF (between −12 and −18 nT) did not cause a substorm onset. The observations show that the period of substorms is not controlled either directly by variations in the solar wind or indirectly by the energy transferred to the magnetosphere from the solar wind. We suggest that a sudden change in the solar wind can trigger a substorm onset if and only if the magnetosphere has reached a state conducive to the generation of substorms. Substorms will occur every ∼3 hours, no matter whether the IMF is continuously southward or fluctuating between southward and northward.

Storm-substorm relationships during the 4 October, 2000 storm. IMAGE global ENA imaging results

Global ion distributions in the 1-200 keV energy range from the main phase of the geomagnetic storm on 4 October 2000 are presented and analyzed. Proton distributions have been obtained by inverting energetic neutral atom (ENA) images from the high energy neutral atom (HENA) instrument on board the IMAGE satellite using a constrained linear inversion technique. The storm is characterized by a 24-hour long main phase where the IMF B z steadily decreases followed by a 2 day recovery. Several substorms occured during the main phase as can be seen from in-situ measurements from geosynchronous satellites (LANL, GOES). Substorm injections during the early main phase, when the dawn to dusk electric field was weak, ocurred on closed trajectories. A strong asymmetric ring current developed as the IMF B z decreased gradually to about -10 nT. A substorm ocurred at about 17:30 UT which injected plasma onto open trajectories with no clear change in the morphology of the partial ring current. As the IMF B z increased towards zero, substorms were observed to inject ions onto closed trajectories. The peak of the ring current moved from L=5 to L=3 during the entire main phase. A preliminary inspection of ∼80-160 keV oxygen ENA fluxes reveal a one order of magnitude increase during the entire main phase, implying that O + contributed significantly to this storm. Rapid decrease followed by decay (∼1 h) was superposed on the gradual increase of the oxygen ENA. Each one of these bursts are associated with a substorm onset. No burst-like features were present in the hydrogen data. In order to quantify the variations in the ring current energy content, the equivalent magnetic disturbance D ENA is calculated for the L≤6 proton distributions using the Dessler-Parker-Sckopke relation. Our calculated D ENA suggests that substorm proton injections did not increase the ring current energy content over the main phase. Together with the fact that the proton ring current was mostly partial, this shows that the dominant ring current energy increase must have been due to increased convection. However, the long-term increase in oxygen ENA fluxes suggests that O + may have been continuously extracted from the ionosphere throughout the main phase and subsequently energized at each substorm dipolarization to give rise to the oxygen ENA bursts. We also discuss implications of strong electric fields in the inner region L<4.

The predictability of the magnetosphere and space weather

Yogi Berra once observed, apparently paraphrasing Niels Bohr, “Prediction is difficult, especially about the future.” n nBerras and Bohrs backgrounds, respectively, in baseball and quantum mechanics, probably prejudiced them, since recent studies show that, at least in geophysics, not everything is as difficult to predict as the path of a knuckle ball or an electron through a double slit. Large-scale magnetospheric activity we believe, is quite predictable, given solar wind conditions. For example, in Figure 1 we show the long-term variation of radiation belt electrons and the Dst index for approximately the last solar cycle. The long-term variation of radiation belt electrons and the Dst index provides evidence that radiation belt electrons and geomagnetic activity on average, have a systematic response to the solar wind.

On the relationships between double‐onset substorm, pseudobreakup, and IMF variation: The 4 September 1999 event

[1] The relationships between double-onset substorm, pseudobreakup, and IMF variation were investigated with magnetic, auroral, and particle observations from space to the ground during 0200-0600 UT on 4 September 1999. There were five consecutive bursts of Pi2 pulsations on the ground during the time of interest. The onset time of ground Pi2s maps to the same variation sequence in the IMF structure seen propagating to the Earth in multiple satellite observations in the upstream region. The comparison of auroral and energetic particle data with IMF observations shows that a sequence of two double-onset substorms intervened by a pseudobreakup appears in two distinct cycles of southward IMF followed by a northward interval. For the first substorm, the first onset begins when the By magnitude declines after the IMF turns southward for about 90 min, and the second onset occurs after northward turning of the IMF accompanied by an increasing By magnitude. The pseudobreakup appears while the IMF turns southward and the By magnitude slightly decreases. For the second substorm, the first onset commences while the IMF remains southward with a steady By magnitude, and the second onset occurs after the IMF becomes strongly northward and the By magnitude decreases instead. These observations can be explained with the two-neutral-point model. The first onset occurs when the IMF turns southward. Reconnection at the near-Earth neutral point first begins on closed field lines within the plasma sheet, and the second onset occurs when the IMF turns northward and reconnection at the distant neutral point ceases and reconnection at the near-Earth neutral point may reach the open flux of the tail lobes. In addition, a decrease in the By magnitude may help reduce magnetotail convection and release all the built-up flux to allow the onset to commence after northward turning of the IMF. If the IMF remains southward, the reduction of magnetotail convection due to a decreasing By would lead to a pseudobreakup instead. In contrast, an increasing B y magnitude would increase magnetotail convection and weaken magnetospheric substorm after the IMF turns northward. Consequently, for the occurrence of double-onset substorms and pseudobreakups, not only the first onset begins spontaneously during steady southward IMF and the second onset appears after northward turning of the IMF but the By change also affects magnetotail convection which may evoke (or abate) the substorm-related activation while the IMF turns southward (or northward).