Relativistic electron flux responses in the inner magnetosphere are investigated for 28 magnetic storms driven by corotating interaction region (CIR) and 27 magnetic storms driven by coronal mass ejection (CME), using data from the Relativistic Electron‐Proton Telescope instrument on board Van Allen Probes from October 2012 to May 2017. In this present study we analyze the role of CIRs and CMEs in electron dynamics by sorting the electron fluxes in terms of averaged solar wind parameters, L‐values, and energies. The major outcomes from our study are the following: (i) At L = 3 and E = 3.4 MeV, for >70% cases the electron flux remains stable, while at L = 5, for ~82% cases it changes with the geomagnetic conditions. (ii) At L = 5, ~53% of the CIR storms and 30% of the CME storms show electron flux increase. (iii) At a given L‐value, the tendency for the electron flux variation diminishes with the increasing energies for both categories of storms. (iv) In case of CIR‐driven storms, the electron flux changes are associated with changes in Vsw and Sym‐H. (v) At L ~ 3, CME storms show increased electron flux, while at L ~ 5, CIR storms are responsible for the electron flux enhancements. (vi) During CME‐ and CIR‐driven storms, distinct electron flux variations are observed at L = 3 and L = 5.
The moderate and intense geomagnetic storms are identified for the first 77 months of solar cycles 23 and 24. The solar sources responsible for the moderate geomagnetic storms are indentified during the same epoch for both the cycles. Solar cycle 24 has shown nearly 80% reduction in the occurrence of intense storms whereas it is only 40% in case of moderate storms when compared to previous cycle. The solar and interplanetary characteristics of the moderate storms driven by coronal mass ejection (CME) are compared for solar cycles 23 and 24 in order to see reduction in geoeffectiveness has anything to do with the occurrence of moderate storm. Though there is reduction in the occurrence of moderate storms, the Dst distribution does not show much difference. Similarly, the solar source parameters like CME speed, mass, and width did not show any significant variation in the average values as well as the distribution. The correlation between VBz and Dst is determined, and it is found to be moderate with value of 0.68 for cycle 23 and 0.61 for cycle 24. The magnetospheric energy flux parameter epsilon (ε) is estimated during the main phase of all moderate storms during solar cycles 23 and 24. The energy transfer decreased in solar cycle 24 when compared to cycle 23. These results are significantly different when all geomagnetic storms are taken into consideration for both the solar cycles.
Zebra stripes are the characteristic structures having repeated hills and valleys in the electron flux intensities observed below L = 3. We delineate the fundamental properties and evolution of electron zebra stripes by modeling advection using time‐dependent electric fields provided by a global magnetohydrodynamics simulation. At the beginning of the simulation, the electrons were uniformly distributed in longitude. Some electrons moved inward due to enhanced westward electric field transients in the premidnight‐postdawn region. The inwardly displaced electrons were confined in a narrow longitudinal range and underwent grad‐B and curvature drifts. For any specific fixed position, the electrons periodically passed through the point with an energy dependent period, giving rise to the hills and valleys in the electron differential flux also known as zebra stripes. The valleys of the zebra stripes are composed of the electrons that underwent outward displacement, or no significant radial displacement. On the nightside, the duskside convection cell is skewed toward dawn in the equatorward of the auroral oval, and the westward electric field becomes dominant in the postdawn region, which results in the inward motion of the electrons. The spatial distribution of the westward electric field is consistent with observation. Zebra stripes are a mixture of the electrons that have and have not experienced inward transport due to solar wind‐inner magnetosphere coupling by way of the ionosphere.
Using Relativistic Electron Proton Telescope measurements onboard Van Allen Probes, the evolution of electron pitch angle distributions (PADs) during the different phases of magnetic storms is studied. Electron fluxes are sorted in terms of storm phase, L value, energy, and magnetic local time (MLT) sectors for 55 magnetic storms from October 2012 through May 2017. To understand the potential mechanisms for the evolution of electron PADs, we fit PADs to a sinusoidal function J 0 sin n ( eq ), where eq is the equatorial pitch angle and n is a real number. The major inferences from our study are (i) at L∼5, the prestorm electron PADs are nearly isotropic (n∼0), which evolves differently in different MLT sectors during the main phase subsequently recovering back to nearly isotropic distribution type during the storm recovery phase; (ii) for E ≤ 3.4 MeV, the main phase electron PADs become more pancake like on the dayside with high n values (>3), while it becomes more flattop to butterfly like on the nightside, (iii) at L = 5, magnetic field strength during the storm main phase enhances during the daytime and decreases during the nighttime. (iv) Conversely, at L ∼3, the electron PADs neither respond significantly to the different phase of the magnetic storm nor reflect any MLT dependence. (v) Main phase, electron fluxes with E <4.2 MeV shows a persistent 90 • maximum PAD with n ranging between 0 and 2, while for E ≥ 4.2 MeV the distribution appears flattop and butterfly like. Our study shows that the relativistic electron PADs depend upon the geomagnetic storm phase and possible underlying mechanisms are discussed in this paper.
The variation of energetic ion composition in the ring current during geomagnetically disturbed intervals has been extensively studied in various aspects since the 1980s. The ring current primarily comprises of ∼1-200 keV ions (mainly O + , He + , and H + ) and electrons in the region of ∼2-7 R E (Daglis et al., 1999;Gonzalez et al., 1994;Hamilton et al., 1988). The energy density of O + ions is usually comparable to the H + ions and sometimes dominates the H + ions during the geomagnetic storms (Daglis et al., 1998;Gloeckler et al., 1985;Hamilton et al., 1988). Earth's plasma sheet is the main region for transporting ions from solar wind and ionospheric origin to the inner magnetosphere, where O + and He + ions increase in comparison with that of H + ions (Nosé et al., 2001).The structure and dynamics of the ring current are determined by these particle populations, which are largely controlled by the time-varying magnetic and electric fields depending on solar wind conditions. Mainly, two types of solar wind drivers are responsible for magnetic storms, that is, Coronal Mass Ejection (CME) and Corotating Interaction Region (CIR) Gonzalez et al., 1999).
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