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Register a new userSpatial structure and temporal evolution of energetic particle injections in the inner magnetosphere during the 14 July 2013 substorm event
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Added by
Kavitha {\\O}stgaard
Publication date
2016
fields
Physics
and research's language is
English
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Recent results by the Van Allen Probes mission showed that the occurrence of energetic ion injections inside geosynchronous orbit could be very frequent throughout the main phase of a geomagnetic storm. Understanding, therefore, the formation and evolution of energetic particle injections is critical in order to quantify their effect in the inner magnetosphere. We present a case study of a substorm event that occurred during a weak storm $textit{ Dst }$ $sim$ -40nT on 14 July 2013. Van Allen Probe B, inside geosynchronous orbit, observed two energetic proton injections within 10min, with different dipolarization signatures and duration. The first one is a dispersionless, short-timescale injection pulse accompanied by a sharp dipolarization signature, while the second one is a dispersed, longer-timescale injection pulse accompanied by a gradual dipolarization signature. We combined ground magnetometer data from various stations and in situ particle and magnetic field data from multiple satellites in the inner magnetosphere and near-Earth plasma sheet to determine the spatial extent of these injections, their temporal evolution, and their effects in the inner magnetosphere. Our results indicate that there are different spatial and temporal scales at which injections can occur in the inner magnetosphere and depict the necessity of multipoint observations of both particle and magnetic field data in order to determine these scales.
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Ground-level enhancements (GLEs) generally accompany with fast interplanetary coronal mass ejections (ICMEs), the shocks driven by which are the effective source of solar energetic particles (SEPs). In the GLE event of 2000 July 14, observations show that a very fast and strong magnetic cloud (MC) is behind the ICME shock and the proton intensity-time profiles observed at 1 au had a rapid two-step decrease near the sheath and MC. Therefore, we study the effect of sheath and MC on SEPs accelerated by an ICME shock through numerically solving the focused transport equation. The shock is regarded as a moving source of SEPs with an assumed particle distribution function. The sheath and MC are set to thick spherical caps with enhanced magnetic field, and the turbulence levels in sheath and MC are set to be higher and lower than that of the ambient solar wind, respectively. The simulation results of proton intensity-time profiles agree well with the observations in energies ranging from $sim$1 to $sim$100 MeV, and the two-step decrease is reproduced when the sheath and MC arrived at the Earth. The simulation results show that the sheath-MC structure reduced the proton intensities for about 2 days after shock passing through the Earth. It is found that the sheath contributed most of the decrease while the MC facilitated the formation of the second step decrease. The simulation also infers that the coordination of magnetic field and turbulence in sheath-MC structure can produce a stronger effect of reducing SEP intensities.
Using in situ measurements and remote-sensing observations, we study a coronal mass ejection (CME) that left the Sun on 9 July 2013 and impacted both Mercury and Earth while the planets were in radial alignment (within $3^circ$). The CME had an initial speed as measured by coronagraphs of 580 $pm$ 20 km s$^{-1}$, an inferred speed at Mercury of 580 $pm$ 30 km s$^{-1}$ and a measured maximum speed at Earth of 530 km s$^{-1}$, indicating that it did not decelerate substantially in the inner heliosphere. The magnetic field measurements made by MESSENGER and {it Wind} reveal a very similar magnetic ejecta at both planets. We consider the CME expansion as measured by the ejecta duration and the decrease of the magnetic field strength between Mercury and Earth and the velocity profile measured {it in situ} by {it Wind}. The long-duration magnetic ejecta (20 and 42 hours at Mercury and Earth, respectively) is found to be associated with a relatively slowly expanding ejecta at 1 AU, revealing that the large size of the ejecta is due to the CME itself or its expansion in the corona or innermost heliosphere, and not due to a rapid expansion between Mercury at 0.45 AU and Earth at 1 AU. We also find evidence that the CME sheath is composed of compressed material accumulated before the shock formed, as well as more recently shocked material.
Magnetic field-line reconnection is a universal plasma process responsible for the conversion of magnetic field energy to the plasma heating and charged particle acceleration. Solar flares and Earths magnetospheric substorms are two most investigated dynamical systems where magnetic reconnection is believed to be responsible for global magnetic field reconfiguration and energization of plasma populations. Such a reconfiguration includes formation of a long-living current systems connecting the primary energy release region and cold dense conductive plasma of photosphere/ionosphere. In both flares and substorms the evolution of this current system correlates with formation and dynamics of energetic particle fluxes. Our study is focused on this similarity between flares and substorms. Using a wide range of datasets available for flare and substorm investigations, we compare qualitatively dynamics of currents and energetic particle fluxes for one flare and one substorm. We showed that there is a clear correlation between energetic particle bursts (associated with energy release due to magnetic reconnection) and magnetic field reconfiguration/formation of current system. We then discuss how datasets of in-situ measurements in the magnetospheric substorm can help in interpretation of datasets gathered for the solar flare.
Forbush decreases (Fds) in galactic cosmic ray intensity are related to interplanetary coronal mass ejections (ICMEs). The parallel diffusion of particles is reduced because the magnetic turbulence level in sheath region bounded by ICMEs leading edge and shock is high. Besides, in sheath and magnetic cloud (MC) energetic particles would feel enhanced magnetic focusing effect caused by the strong inhomogeneity of the background magnetic field. Therefore, particles would be partially blocked in sheath-MC structure. Here, we study two-step Fds by considering the magnetic turbulence and background magnetic field in sheath-MC structure with diffusion coefficients calculated with theoretical models, to reproduce the Fd associated with the ground-level enhancement event on 2000 July 14 by solving the focused transport equation. The sheath and MC are set to spherical caps that are portions of spherical shells with enhanced background magnetic field. Besides, the magnetic turbulence levels in sheath and MC are set to higher and lower than that in ambient solar wind, respectively. In general, the simulation result conforms to the main characteristics of the Fd observation, such as the pre-increase precursor, amplitude, total recovery time, and the two-step decrease of the flux at the arrival of sheath and MC. It is suggested that sheath played an important role in the amplitude of Fd while MC contributed to the formation of the second step decrease and prolonged the recovery time. It is also inferred that both magnetic turbulence and background magnetic field in sheath-MC structure are important for reproducing the observed two-step Fd.
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