Edmund Spencer

The effect of changing solar wind conditions on the inner magnetosphere and ring current: A model data comparison

We investigate the rate at which the open drift paths in the near-Earth magnetosphere convert to closed paths during events with a sudden northward turning of the interplanetary magnetic field (IMF Bz) after the peak of a geomagnetic storm. The geomagnetic storm on 17 August 2001 with an abrupt turning of the IMF Bz after the peak in SYM-H index is chosen in this study. The Block-Adaptive-Tree Solar-Wind Roe-Type Upwind Scheme model along with the Fok Ring Current (FRC) model available at Community Coordinated Modeling Center is used to model this event. The unique movie maps of the worldwide magnetometer stations are used to compare with the numerical results. The results indicate that ground magnetic disturbance remains asymmetric for some time after the start of the recovery phase even for a storm with abrupt northward turning of the IMF Bz. FRC simulation results suggest that the flow-out losses decrease under weakened magnetospheric convection but at a rate slower than the change in IMF Bz. These results indicate that the flow-out losses rapidly become smaller as the IMF Bz turns northward during the early recovery phase of a storm and the contribution of the tail current to the SYM-H index is important.

The dynamics of geomagnetic substorms with the WINDMI model

The global dynamics of substorms are controlled by several key magnetospheric parameters. In this work we obtain quantitative measures of these parameters from a low-order nonlinear model of the nightside magnetosphere called WINDMI. The model uses solar wind and IMF measurements from the ACE spacecraft as input into a system of 8 nonlinear ordinary differential equations. The state variables of the differential equations represent the energy stored in the geomagnetic tail, central plasma sheet, ring current and field-aligned currents. The output from the model is the geomagnetic westward auroral electrojet (AL) index and the Dst index. Intermediate variables of the model are the plasma sheet pressure, geotail current, cross-tail electric field, parallel ion velocity and the pressure gradient current. The values of these variables are controlled by physical parameters of the model, consisting of spatially averaged quantities that are analogous to electric circuit elements. We tune the model to re-produce substorm events, comparing model capability against observations of auroral brightening and the auroral electrojet indices AL from WDC Kyoto and SML from SuperMAG. The model is capable of capturing events within a 10–12-min interval of occurrence, with level of activity comparable to the measured indices.

A System On Chip design for fast time domain impedance spectroscopy

Impedance spectroscopy is a powerful technique that can be employed to determine the physical properties of different materials. In principle the technique can be implemented in the time domain using pulsed signal excitation rather than sweeping across frequencies. The advantage is particularly apparent when the measuring instrument is either traveling through the material medium rapidly, as in the case of a satellite moving through space plasma, or if the medium is for example a fluid that flows past an instrument quickly. Here we describe a Time Domain Impedance Probe (TDIP) circuit design that is used to measure the absolute density of ionospheric plasmas. A preliminary version of this instrument was flown on a sounding rocket, but here we outline the system and circuit design that is being implemented for a Low Earth Orbit (LEO) micro-satellite. The design employs a bridge architecture together with a software adaptive filter and LMS algorithm for fast calibration and data compression. We propose that the design can be generalized, and we present a System On Chip (SOC) concept based on the time domain architecture. The proposed concept appears to be well suited to ultra-fast time domain spectroscopic measurements, but does have some inherent limitations such as increased noise. We suggest that this unavoidable shortcoming can be somewhat mitigated through repetitive pulsing and averaging during the measurement process.

First results from a time domain impedance probe for measuring plasma properties in the ionosphere

A new Time Domain Impedance Probe (TDIP) is presented in this paper. The new instrument is able to make measurements of absolute electron density and electron neutral collision frequency in the ionosphere at temporal and spatial resolutions not previously attained. A single measurement is made in 100 microseconds, which yields an instantaneous spatial resolution of 0.1 meters for sounding rocket experiments. A prototype of this instrument was integrated into the payload of a NASA USIP sounding rocket launched out of Wallops Island on March 1 2016. The sounding rocket launched at 8:50 am and reached an reached an altitude of 170 km, passing through the D and E and F layers of the ionosphere. The TDIP was active for 206 seconds during the flight. Here we describe the instrument, and present some time domain data obtained from the sounding rocket experiment. A 6 Volt amplitude Gaussian derivative excitation was applied to a dipole probe structure, and the current through the probe terminals measured with a balanced active bridge circuit. The time domain current response was sampled at 5 MS/s, at 12 bit resolution. In the course of the flight, the instrument measured what appeared to be a highly nonlinear response of the plasma because of the large input voltage signal applied. These are the first measurements of this type of response, to our knowledge. Post-flight laboratory calibration indicated that the instrument worked correctly through the flight. Further modeling, simulation and theoretical work needs to be performed to understand and interpret the measurements.

Collisionless resistivity in a bifurcated current sheet

The collisionless resistivity due to charged particle chaos in spatially inhomogeneous magnetic fields is calculated for two frequently observed magnetotail current sheets, the X line, and a bifurcated current sheet (BCS) over varying strengths of cross-tail electric field. The calculation is done for two charged species, protons and O+ ions, found in the magnetotail specially during active times. Chaotic behavior of the particles is studied for the chaos parameter κ≈1defined by Buchner and Zelenyi (1989) as the square root of minimum radius of curvature to the maximum particle gyroradius. The surface of section plot and maximal Lyapunov exponents are analyzed to compare the particle behavior in the two magnetic field topologies. It is found that the particle behavior in a BCS is chaotic around the two humps located at ±z0 and that the configuration is more chaotic than the X line (maximum Lyapunov exponent for X line is around 0.25 compared to 0.34 for BCS). The collisionless resistivity is calculated using the technique developed by Numata and Yoshida (2003). The method relies on a phenomenological equivalence between the particle loss rate from the chaos region and rate of change of momentum in the chaos region under steady state conditions. Rapid decay of the particles from the chaos region is modeled with exponential fits, and it is found that a double exponent is needed for O+ ions in an X line and for both species in the BCS. The resistivity thus calculated is found to be 9–10 orders of magnitude higher than the Spitzer resistivity.