Xiaoyan Zhou

THEMIS observations of ULF wave excitation in the nightside plasma sheet during sudden impulse events

[1]xa0Sudden impulses (SIs) are an important source of ultra low frequency (ULF) wave activity throughout the Earths magnetosphere. Most SI-induced ULF wave events have been reported in the dayside magnetosphere; it is not clear when and how SIs drive ULF wave activity in the nightside plasma sheet. We examined the ULF response of the nightside plasma sheet to SIs using an ensemble of 13 SI events observed by THEMIS (Timed History of Events and Macroscale Interactions during Substorms) satellites (probes). Only three of these events resulted in ULF wave activity. The periods of the waves are found to be 3.3, 6.0, and 7.6u2009min. East-west magnetic and radial electric field perturbations, which typically indicate the toroidal mode, are found to be stronger and can have phase relationships consistent with standing waves. Our results suggest that the two largest-amplitude ULF responses to SIs in the nightside plasma sheet are tailward-moving vortices, which have previously been reported, and the dynamic response of cross-tail currents in the magnetotail to maintain force balance with the solar wind, which has not previously been reported as a ULF wave driver. Both mechanisms could potentially drive standing Alfven waves (toroidal modes) observed via the field-line resonance mechanism. Furthermore, both involve frequency selection and a preference for certain driving conditions that can explain the small number of ULF wave events associated with SIs in the nightside plasma sheet.

Supercriticality of ICME and CIR shocks

Interplanetary coronal mass ejection (ICME) and corotating interaction region (CIR) shocks are characterized in terms of supercriticality introduced by Edmiston and Kennel (1984) to classify shocks based on whether dissipation is provided by electron resistivity alone or also requires ion viscosity. The condition for determining supercriticality is a critical Mach number, MC, a function of θBn, the angle between the upstream magnetic field, B, and the normal to the shock surface, n, and β, the ratio of the plasma and magnetic pressures. The criterion was subsequently revised by Kennel (1987) to include dissipation by electron thermal as well as electrical conductivity. Two early separate studies of ICME and CIR shocks motivated our investigation that included several improvements. We use Kennel (1987) and shocks identified by WIND near 1u2009AU and by Ulysses near 5u2009AU from the same solar cycle to provide Occurrence Probability Distributions and statistical information for all parameters. We answer three questions (1) Is the supercriticality of ICME and CIR shocks different? (2) If so, why? (3) Does the latter MC criterion change the answers? Our conclusions are (1) about two thirds of CIR shocks are supercritical as compared to one third of ICME shocks, (2) although ICME shock speeds are typically higher than CIR shocks, the fast-mode wave speeds are even higher at 1u2009AU than that of CIR shocks at ~5u2009AU causing a reduction in Mach numbers, and (3) CIR shocks are also more supercritical than ICME shocks using both criteria with slight differences.

Shock aurora: Field‐aligned discrete structures moving along the dawnside oval

Generatedby interplanetary shocks or solarwindpressurepulses, shock aurorahas transient, global, and dynamic significances and provides a direct manifestation of the solar wind-magnetosphere-ionosphere interaction. As a part of a series of studies of the shock aurora, this paper focuses on the interaction at the morning magnetopause and its auroral manifestation at ~06 magnetic local time, where the velocity and magnetic field shears dominate the interaction. Flow shears can generate wave-like structures inside a viscous boundary layer or even larger-scale vortices. These structures couple to the ionosphere via quasi-static field-aligned currents or via kinetic Alfvén waves. Potential drops along field-aligned filaments may be generated accelerating electrons to form auroral manifestations of the structures. A shock aurora event at dawnside is used to test this scenario. The findings include moving auroral streaks/rays that have a vertical profile from red (at ~250 km altitude) to purple (at ~100 km). The streaks moved antisunward along the poleward boundary of the oval at an ionospheric speed of ~3 km s . It was mapped to the magnetopause flank at ~133 km s , which was consistent with the observed speed of the magnetopause surface waves generated by the Kelvin-Helmholtz instability. The calculated field-aligned potential drop using Haerendel’s analytic model was ~5 kV that reasonably explained the observations. The results support the above scenario and reveal that magnetic and velocity shears at the flanks of the magnetospause may be the main cause of the fast moving shock aurora streaks.

Development of a near‐infrared balloon‐borne camera for dayside and sunlit auroral observations

Imaging aurora in daylight is a difficult and challenging task. The brightness of the sunlit atmosphere overwhelms the auroral emissions at visible wavelengths. Modeling of atmospheric brightness suggests that the contrast between auroral brightness and sky brightness makes it possible to image the aurora at near infrared (NIR) wavelengths from sufficient altitudes. Preliminary experiments confirmed that the auroral N2+ Meinel emissions at about 1100u2009nm are bright enough to be extracted from atmospheric background brightness during daylight at about 40u2009km of a balloon altitude, which lead to the development of a high-performance NIR InGaAs camera that can be flown on a high-altitude and long-duration balloon. Auroral observations from such a platform are highly accommodated to current space missions (such as THEMIS/ARTEMIS, MMS, Cluster, Geotail, DMSP) and many ground-based measurements and will enhance the science return significantly.