arXiv subjects

D. G. Mitchell

Publications and source records attributed to D. G. Mitchell.

25 records · Page 2Linked to original sources

arXiv2019

Seed Population Pre-Conditioning and Acceleration Observed by Parker Solar Probe

A series of solar energetic particle (SEP) events were observed at Parker Solar Probe (PSP) by the Integrated Science Investigation of the Sun (\ISOIS) during the period from April 18, 2019 through April 24, 2019. The PSP spacecraft was located near 0.48 au from the Sun on Parker spiral field lines that projected out to 1 au within $\sim 25^\circ$ of near Earth spacecraft. These SEP events, though small compared to historically large SEP events, were amongst the largest observed thus far in the PSP mission and provide critical information about the space environment inside 1 au during SEP events. During this period the Sun released multiple coronal mass ejections (CMEs). One of these CMEs observed was initiated on April 20, 2019 at 01:25 UTC, and the interplanetary CME (ICME) propagated out and passed over the PSP spacecraft. Observations by the Electromagnetic Fields Investigation (FIELDS) show that the magnetic field structure was mostly radial throughout the passage of the compression region and the plasma that followed, indicating that PSP did not directly observe a flux rope internal to the ICME, consistent with the location of PSP on the ICME flank. Analysis using relativistic electrons observed near Earth by the Electron, Proton and Alpha Monitor (EPAM) on the Advanced Composition Explorer (ACE) demonstrates the presence of electron seed populations (40--300 keV) during the events observed. The energy spectrum of the \ISOIS~ observed proton seed population below 1 MeV is close to the limit of possible stationary state plasma distributions out of equilibrium. \ISOIS~ observations reveal the \revise{enhancement} of seed populations during the passage of the ICME, which \revise{likely indicates a key part} of the pre-acceleration process that occurs close to the Sun.

astro-ph.SR↗

arXiv2019

Energetic Particle Observations from Parker Solar Probe using Combined Energy Spectra from the IS$\odot$IS Instrument Suite

The Integrated Science Investigations of the Sun (IS$\odot$IS) instrument suite includes two Energetic Particle instruments: EPI-Hi, designed to measure ions from ~1-200 MeV/nuc, and EPI-Lo, designed to measure ions from ~20 keV/nuc to ~15 MeV/nuc. We present an analysis of eight energetic proton events observed across the energy range of both instruments during PSP's first two orbits in order to examine their combined energy spectra. Background corrections are applied to help resolve spectral breaks between the two instruments and are shown to be effective. In doing so we demonstrate that, even in the early stages of calibration, IS$\odot$IS is capable of producing reliable spectral observations across broad energy ranges. In addition to making groundbreaking measurements very near the Sun, IS$\odot$IS also characterizes energetic particle populations over a range of heliocentric distances inside 1 au. During the first two orbits, IS$\odot$IS observed energetic particle events from a single corotating interaction region (CIR) at three different distances from the Sun. The events are separated by two Carrington rotations and just 0.11 au in distance, however the relationship shown between proton intensities and proximity of the spacecraft to the source region shows evidence of the importance of transport effects on observations of energetic particles from CIRs. Future IS$\odot$IS observations of similar events over larger distances will help disentangle the effects of CIR-related acceleration and transport. We apply similar spectral analyses to the remaining five events, including four that are likely related to stream interaction regions (SIRs) and one solar energetic particle (SEP) event.

physics.space-ph↗

arXiv2019

Plasma pressures in the heliosheath from Cassini ENA and Voyager 2 measurements: Validation by the Voyager 2 heliopause crossing

We report "ground truth", 28-3500 keV in-situ ion and 5.2-55 keV remotely sensed ENA measurements from Voyager 2/Low Energy Charged Particle (LECP) detector and Cassini/Ion and Neutral Camera (INCA), respectively, that assess the components of the ion pressure in the heliosheath. In this process, we predict an interstellar neutral hydrogen density of ~0.12 cm-3 and an interstellar magnetic field strength of ~0.5 nT upstream of the heliopause in the direction of V2, i.e. consistent with the measured magnetic field and neutral density measurements at Voyager 1 from August 2012, when the spacecraft entered interstellar space, to date. Further, this analysis results in an estimated heliopause crossing by V2 of ~119 AU, as observed, suggesting that the parameters deduced from the pressure analysis are valid. The shape of the >5.2 keV ion energy spectra play a critical role towards determining the pressure balance and acceleration mechanisms inside the heliosheath.

physics.space-ph↗

arXiv2016

Spatial structure and temporal evolution of energetic particle injections in the inner magnetosphere during the 14 July 2013 substorm event

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.

physics.space-ph↗

arXiv2016

Plutos interaction with its space environment: Solar Wind, Energetic Particles & Dust

The New Horizons spacecraft carried three instruments that measured the space environment near Pluto as it flew by on 14 July 2015. The Solar Wind Around Pluto instrument revealed an interaction region confined sunward of Pluto to within about 6 Pluto radii. The surprisingly small size is consistent with a reduced atmospheric escape rate as well as a particularly high solar wind flux. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) observations suggested ions are accelerated and-or deflected around Pluto. In the wake of the interaction region PEPSSI observed suprathermal particle fluxes about one tenth the flux in the interplanetary medium, increasing with distance downstream. The Student Dust Counter, which measures radius greater than 1.4 um grains, detected 1 candidate impact from 5days before to 5 days after closest approach, indicating an upper limit for the dust density in the Pluto system of 4.6 per cubic km.

astro-ph.EP↗

arXiv2015

Nanodust detection between 1 and 5 AU by using Cassini wave measurements

The solar system contains solids of all sizes, ranging from km-size bodies to nano-sized particles. Nanograins have been detected in situ in the Earth's atmosphere, near cometary and giant planet environments, and more recently in the solar wind at 1 AU. These latter nano grains are thought to be formed in the inner solar system dust cloud, mainly through collisional break-up of larger grains and are then picked-up and accelerated by the magnetized solar wind because of their large charge-to-mass ratio. In the present paper, we analyze the low frequency bursty noise identified in the Cassini radio and plasma wave data during the spacecraft cruise phase inside Jupiter's orbit. The magnitude, spectral shape and waveform of this broadband noise is consistent with the signature of nano particles impinging at nearby the solar wind speed on the spacecraft surface. Nanoparticles were observed whenever the radio instrument was turned on and able to detect them, at different heliocentric distances between Earth and Jupiter, suggesting their ubiquitous presence in the heliosphere. We analyzed the radial dependence of the nano dust flux with heliospheric distance and found that it is consistent with the dynamics of nano dust originating from the inner heliosphere and picked-up by the solar wind. The contribution of the nano dust produced in asteroid belt appears to be negligible compared to the trapping region in the inner heliosphere. In contrast, further out, nano dust are mainly produced by the volcanism of active moons such as Io and Enceladus.

astro-ph.EP↗

arXiv2013

Multi-spectral simultaneous diagnosis of Saturns aurorae throughout a planetary rotation

From the 27th to the 28th January 2009, the Cassini spacecraft remotely acquired combined observations of Saturns southern aurorae at radio, ultraviolet and infrared wavelengths, while monitoring ion injections in the middle magnetosphere from energetic neutral atoms. Simultaneous measurements included the sampling of a full planetary rotation, a relevant timescale to investigate auroral emissions driven by processes internal to the magnetosphere. In addition, this interval coincidently matched a powerful substorm-like event in the magnetotail, which induced an overall dawnside intensification of the magnetospheric and auroral activity. We comparatively analyze this unique set of measurements to reach a comprehensive view of kronian auroral processes over the investigated timescale. We identify three source regions in atmospheric aurorae, including a main oval associated with the bulk of Saturn Kilometric Radiation (SKR), together with polar and equatorward emissions. These observations reveal the co-existence of corotational and sub-corototational dynamics of emissions associated with the main auroral oval. Precisely, we show that the atmospheric main oval hosts short-lived sub-corotating isolated features together with a bright, longitudinally extended, corotating region locked at the southern SKR phase. We assign the susbtorm-like event to a regular, internally driven, nightside ion injection possibly triggerred by a plasmoid ejection. We also investigate the total auroral energy budget, from the power input to the atmosphere, characterized by precipitating electrons up to 20 keV, to its dissipation through the various radiating processes. Finally, through simulations, we confirm the search-light nature of the SKR rotational modulation and we show that SKR arcs relate to isolated auroral spots. The resulting findings are discussed in the frame of pending questions.

astro-ph.EP↗