New light shed on dayside Aurora
Top image: Dayside aurora observed from the auroral station in Adventdalen. Photo: Jeff Holmes/UNIS
The dayside aurora is less studied and less well understood than the nightside counterpart. In a doctoral thesis by UNIS PhD candidate Jeffrey Holmes, further understanding of precipitating protons in the dayside aurora is revealed. Holmes will defend his thesis on Friday 29 August at the University of Oslo.
28 August 2014
Press release from UNIS and the University of Oslo
The dayside aurora, the portion of the auroral oval facing the Sun, is historically less studied and less well understood than the nightside counterpart.
An observational project using spectrometers in Svalbard has shown that the variation of auroral proton energy in the cusp can be tracked by observing the spectral Doppler profile of auroral hydrogen emissions from a minimum of two observation locations. In addition, sudden auroral brightening that propagates along the auroral oval owing to shocks in the solar wind is associated with oscillating field-aligned current systems and double pulses in hydrogen aurora.
The Protonics project, which Jeff Holmes’ PhD is part of, is an effort to further understand the spatio-temporal dynamics of dayside auroral hydrogen emissions, also known as dayside proton aurorae. Spectrometers measuring dayside Balmer α (Hα) and Balmer β (Hβ) were deployed to two locations in Svalbard; Longyearbyen and Ny-Ålesund.
Measured hydrogen Doppler profiles were analysed via a Monte Carlo model of proton precipitation, resulting in an estimate of characteristic energy of the precipitating proton/hydrogen population. The difference in energy found between the two stations is interpreted as an ionospheric signature of magnetic merging near the magnetopause.
Initially, a significant energy difference was discovered in two cases. However, weak emissions required spectral scans from the two instruments to be separately averaged for roughly two hours to produce the result. A third case featuring a stronger proton precipitation event was found, resulting in a statistically significant difference in energy with averaging on the order of minutes. This third case is the first statistically significant ground-based detection of the ion velocity filter effect in the dayside hydrogen aurora.
A natural extension of the project was to investigate the relative occurrence of electron and proton aurora under the influence of solar wind shocks across the boreal auroral zone. Since this study required areal data coverage much larger than the vicinity of Svalbard, data from meridian scanning photometers (MSP) in Canada, Greenland and Svalbard were combined and compared with large-scale UV auroral images from the Polar spacecraft. Analysis of MSP data for events previously studied solely using space-based imagery added needed spatio-temporal resolution. Shock aurora propagation times were refined, and agreed with previous results to within uncertainties. Furthermore, the majority of instruments detected low energy discrete auroral arcs poleward of diffuse, higher energy proton and electron aurora. Two-pulse proton aurora onset sequences were also observed.
Svalbard is an excellent place to make ground-based spectroscopic observations of the dayside aurora, given its long periods of winter darkness. Also, Svalbard’s location relative to the geomagnetic North Pole places it underneath the magnetospheric cusp, a region where solar wind particles can easily precipitate into the atmosphere and create dayside aurora. Spectroscopic data used in the study were obtained at Ny-Ålesund and both Nordlysstasjonen (The Auroral Station) and the Kjell Henriksen Observatory near Longyearbyen.
Jeffrey Holmes will defend his PhD thesis, entitled “The Protonics project: distributed observations of auroral dayside Doppler-shifted hydrogen emissions” on Friday 29 August 2014 at 13:15 in Lille fysiske auditorium, Fysikkbygningen at the University of Oslo.
He will hold a trial lecture entitled “Fifty years of magnetospheric substorm research: historical review and recent developments”, in the same auditorium at 10:15 on Friday 29 August.
About the candidate:
Jeffrey Holmes received a bachelor of science in physics from the University of Texas at Austin in 1998, and a master of science in space physics from the University of Alaska Fairbanks in 2004. Jeffrey is a Texas native.