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C T Russell

Publications and source records attributed to C T Russell.

9 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2022

Kelvin-Helmholtz Vortices as an Interplay of Magnetosphere-Ionosphere Coupling

The solar wind-magnetosphere interaction drives diverse physical processes on the flanks of Earth’s magnetopause, and in turn these processes couple to the ionosphere. We investigate simultaneous multipoint in-situ spacecraft and ground-based measurements to determine the role of Kelvin-Helmholtz waves at the Earth’s magnetopause and the low-latitude boundary layer in the magnetosphere-ionosphere coupling process. Nonlinear Kelvin-Helmholtz waves develop into flow vortices that twist and/or shear flux tube magnetic fields, thereby generating localized field-aligned currents. Kelvin-Helmholtz vortices on the dusk (dawn) flanks of the magnetosphere generate clockwise (counter-clockwise) rotations and upward (downward) field-aligned currents inside the flux tubes, consistent with the region-1 field-aligned current. We present in-situ MMS and Cluster spacecraft observations of Kelvin-Helmholtz vortices at the magnetopause that map to the poleward edge of the auroral regions. The FAST spacecraft and the ground-based magnetometers from which spherical elementary currents (acting as a proxy for vertical currents) can be calculated observe corresponding field-aligned current signatures. This study demonstrates the role played by the Kelvin-Helmholtz waves in linking magnetopause boundary fluctuations to ionospheric phenomena.

K-J Hwang↗
NASA NTRS2022

Electron Energization and Thermal to Non-Thermal Energy Partition During Earth's Magnetotail Reconnection

Electrons in earth's magnetotail are energized significantly both in the form of heating and in the form of acceleration to non-thermal energies. While magnetic reconnection is considered to play an important role in this energization, it still remains unclear how electrons are energized and how energy is partitioned between thermal and non-thermal components. Here, we show, based on in situ observations by NASA's magnetospheric multiscale mission combined with multi-component spectral fitting methods, that the average electron energy ε¯(or equivalently temperature) is substantially higher when the locally averaged electric field magnitude |𝐸| is also higher. While this result is consistent with the classification of “plasma-sheet” and “tail-lobe” reconnection during which reconnection is considered to occur on closed and open magnetic field lines, respectively, it further suggests that a stochastic Fermi acceleration in 3D, reconnection-driven turbulence is essential for the production and confinement of energetic electrons in the reconnection region. The puzzle is that the non-thermal power-law component can be quite small even when the electric field is large and the bulk population is significantly heated. The fraction of non-thermal electron energies varies from sample to sample between ∼20% and ∼60%, regardless of the electric field magnitude. Interestingly, these values of non-thermal fractions are similar to those obtained for the above-the-looptop hard x-ray coronal sources for solar flares.

M Oka↗
NASA NTRS2022

The EDR Inflow Region of a Reconnecting Current Sheet in the Geomagnetic Tail

On 6 July 2017, the four Magnetospheric Multiscale spacecrafts were positioned within an electron diffusion region (EDR) just northward of a reconnection X line. The EDR was identified by electron crescent distributions, out-of-plane current, and energy conversion. From this position, the three spacecrafts closest to the X line (within about three electron inertial lengths) were able to accurately measure the reconnection electric field and the electron inflow velocity. The reconnection rates derived from the electric field and inflow velocity measurements agree with theoretical estimates (0.11–0.17) and a previous measurement of EM in a tail reconnection event on 11 July 2017.

Energy conversion↗
NASA NTRS2021

Microscale Processes Determining Macroscale Evolution of Magnetic Flux Tubes along Earth’s Magnetopause

An important process affecting solar wind-Earth’s magnetosphere coupling is non-steady dayside magnetic reconnection, observationally evidenced by a flux-transfer-event (FTE) that shows a bipolar variation of the magnetic field component normal to the magnetopause. FTEs often consist of two interlinked flux tubes, but, local kinetic processes between the flux tubes are not understood in the context of the FTE structuring, evolution, and impact. An FTE observed by MMS on 18 December 2017 consisted of two flux tubes of different topology. One includes field lines with ends connected to the northern and southern hemispheres while the other includes field lines with both ends connected to the magnetosheath. Reconnection occurring at the flux-tube interface indicates how interacting flux tubes evolve into a flux rope with helical magnetic topology that is either closed or open. This study demonstrates a new aspect of how micro-to meso-scale dynamics occurring within FTEs determines their macroscale characteristics and evolution.

Solar-terrestrial interactions↗
NASA NTRS2021

An Encounter with the Ion and Electron Diffusion Regions at a Flapping and Twisted Tail Current Sheet

We analyze data returned by the Magnetospheric Multiscale mission (MMS) constellation during a rapid (~1.5 s) traversal of a flapping and reconnecting current sheet (CS) in the near-Earth magnetotail (X ~ -20 RE). The CS was highly tilted, with its normal pointing strongly duskward. Its extreme thinness was confirmed by a curvature analysis of the magnetic field lines. The event was associated with a guide field of 8% of the reconnecting components. From the pitch angle distributions of low-energy electrons we infer that the crossing occurred earthward of the X-line. Traveling practically normal to the CS, MMS encountered an ion diffusion region (IDR) in which was embedded an electron diffusion region (EDR). IDR signatures included breaking of the ion frozen-in condition in the presence of Hall B and E fields. EDR signatures included a strong out of-plane current associated with a super-Alfvenic electron jet, evidence of positive energy transfer, and a temperature anisotropy (TeII > Te1) which disappeared at the field reversal. Derived scale sizes normal to the CS are: ~6.9 de (EDR) and ~0.4 di (IDR) (40 and 100 km). We estimate the average dimensionless reconnection rate as 0.077 + 0.050. The observations and inferences are supported by PIC numerical simulations. We find very good agreement in the reconnection rates. We also discuss the effects of asymmetries in the density, temperature and magnetic field strength on the Hall fields and length of the outflow jets. The event is associated with a substorm onset which began 7 min after the MMS observations.

C J Farrugia↗
NASA NTRS2019

The Solar Clock

The Sun is powered by a very stable source of fusion energy in its core that radiates that energy outward in a constant flow. Yet it has a cycle of magnetic dynamo activity whose strength and duration are variable. This variability, which affects the Earth's ‘space climate', points to temporal changes in the convective and diffusive transport of magnetic flux above the tachocline, where the flux is generated. The longest record we have of this variability is the time series of Sunspot Numbers (SSN). This record suggests that the interior of the Sun follows a clocklike magnetic flux production cycle with a length of close to 11.05 yr. The variations in sunspot cycle duration, as well as the sunspot number rise and fall times, their hemispheric asymmetries, and the maximum sunspot numbers of the individual cycles, are likely produced in the process of the magnetic flux transport. Helioseismology continues to shed more light on the convection zone variabilities, including the ‘torsional oscillations' that seem to have a special connection to the emerging strong magnetic fluxes that produce the sunspots. These new observations may eventually lead to an explanation for the surprisingly good correlation between the rate of sunspot appearance and the maximum sunspot number, and to a better understanding of the relationship between the solar dynamo and the sunspot number cycle. Better understanding of the polar regions awaits long-term monitoring with a polar solar mission. Better predictions of nearterm space weather could be obtained from a permanent L5 monitor.

C T Russell↗
NASA NTRS2015

Vestas Pinaria Region: Original Basaltic Achondrite Material Derived from Mixing Upper and Lower Crust

Analysis of data from the Dawn mission shows that the Pinaria region of Vesta spanning a portion of the rim of the Rheasilvia basin is bright and anhydrous. Reflectance spectra, absorption band centers, and their variations, cover the range of pyroxenes from diogenite-rich to howardite and eucrite compositions, with no evidence of olivine in this region. By examining band centers and depths of the floor, walls and rims of six major craters in the region, we find a lane of diogenite-rich material next to howardite-eucrite material that does not follow the local topography. The source of this material is not clear and is probably ejecta from post-Rheasilvia impacts. Material of a howardite-eucrite composition originating from beyond the Rheasilvia basin is evident on the western edge of the region. Overall, the Pinaria region exposes the complete range of basaltic achondrite parent body material, with little evidence of contamination of non-basaltic achondrite material. With both high reflectance and low abundance of hydrated material, this region of Vesta may be considered the "Pinaria desert".

Vesta↗
NASA NTRSDate not supplied

STEREO Observations of Stream Interaction Regions in 2007-2016:Relationship with Heliospheric Current Sheets, Solar Cycle Variations, and Dual Observations

We have conducted a survey of 575 slow-to-fast stream interaction regions (SIRs) using STEREOA/B data in 2007-2016 (Jian et al., 2019). Among 518 well-defined SIRs, 54% are associated with heliospheric current sheet (HCS) crossings, and 34% are without any HCS crossing. The other 12% of the SIRs often occur in association with magnetic sectors shorter than 3 days. The SIRs with HCS crossings have slightly slower speeds but higher maximum number densities, magnetic field strengths, dynamic pressures, and total pressures than the SIRs without an HCS. The iron charge state is higher throughout the SIRs with an HCS than the SIRs without an HCS, by about 1/3 charge unit. In contrast with the comparable phases of solar cycle 23,slightly more SIRs and higher recurrence rates are observed in the years 2009-2016 of cycle 24,with a lower HCS association rate, possibly attributed to persistent equatorial coronal holes and more pseudostreamers in this recent cycle. The solar wind speed, peak magnetic field and pressures of SIRs are all lower in this cycle, but the weakening is less than for the comparable background solar wind parameters. Before STEREO B lost contact in 2014 October, 151 SIR pairs were observed by the twin spacecraft. Of the dual observations, the maximum speed is the best correlated of the plasma parameters. We have obtained a sample of plasma parameter differences analogous to those that would be observed for a mission at Lagrange point 4 or 5.By studying several cases with large discrepancies between the dual observations, we have also investigated the effects of HCS relative location, tilt of stream interface, and small transients on the SIR properties.

Lan K Jian↗