arXiv subjects

Björn Gustavsson

Publications and source records attributed to Björn Gustavsson.

3 recordsLinked to original sources

arXiv2026

Estimating sub-frame time differences in camera image sequences

Some optical measurements require relative timing of intensity variations with accuracy much finer than the camera frame period. One motivating example is dynamic aurora, where different prompt emissions are expected to originate from different altitude regions and can therefore have millisecond-scale relative delays caused by finite energetic-electron velocities and other electron-transport effects. These delays are predicted to be a small fraction of the frame duration of typical auroral video cameras. We present a cross-spectral technique for estimating the relative delay between two time-varying optical intensity signals recorded by one or more image sensors. The method is validated with a calibration device that generates two pseudorandomly pulsed optical emissions with a known relative delay, recorded using a consumer smartphone camera. For the tested recordings, the method estimates relative delays between image-sensor regions with better than $50$~$μ$s accuracy. Although developed for high-frame-rate auroral imaging, the technique has numerous other imaging applications, including camera timing calibration and measurements of time-varying optical signals. The single-camera tests demonstrate that the method can characterize sub-frame timing differences across an image sensor, such as those produced by rolling-shutter readout. The same analysis applies to separate cameras when they observe the same time-varying signal and are synchronized to a shared clock.

physics.space-ph↗

arXiv2026

Optical and Radar Observations of the February 2025 Falcon 9 Upper-Stage Re-entry

We investigate the February 19, 2025, re-entry of a Falcon 9 upper stage using optical observations from 43 meteor cameras across central Europe together with radar detections of re-entry plasma obtained with the 32.55 MHz SIMONe Germany multistatic radar system. Optical observations of fragment emissions between 85 and 36 km altitude were used to reconstruct 30 fragment trajectories, identify two main fragment families, and fit ballistic trajectories to estimate kinetic energy loss per unit mass. The optical detection-height distribution peaks near 60 km with a standard deviation of 10 km, and both optical and radar signatures occur in the same broad altitude region as the maximum kinetic-energy loss. Radar echoes were detected at altitudes between 55 and 75 km, and the radar-derived positions are consistent with those obtained from optical observations. Two distinct radar echo types associated with the re-entry plasma were identified: (1) specular trail echoes from overdense wake plasma, with radar cross-sections (RCS) of up to 60 dBsm, and (2) short-lived non-specular trail echoes with RCS values of 20--30 dBsm, exhibiting a delay of 1--2 s compared to optical signatures. The characteristic decay time of both echo types is approximately 1 s. In the radar-echo altitude range, the estimated Knudsen numbers for meter-scale fragments are well below unity, consistent with continuum-flow conditions and shock-driven plasma production rather than ordinary meteor-like impact ionization. These serendipitous radar observations demonstrate that the atmospheric re-entry of other spacecraft, including objects smaller than the Falcon 9 upper stage such as Starlink satellites, may likewise be detectable using comparable multistatic meteor radar systems deployed globally.

physics.space-ph↗

arXiv2016

Eastward-expanding auroral surges observed in the post-midnight sector during a multiple-onset substorm

We present three eastward-expanding auroral surge (EEAS) events that were observed intermittently at intervals of about 15 min in the post-midnight sector (01:55-02:40 MLT) by all-sky imagers and magnetometers in northern Europe. It was deduced that each surge occurred just after each onset of a multiple-onset substorm, which was small-scale and did not clearly expand westward, because they were observed almost simultaneously with Pi 2 pulsations at the magnetic equator and magnetic bay variations at middle-to-high latitudes associated with the DP-1 current system. The EEASs showed similar properties to omega bands or torches reported in previous studies, such as recurrence intervals of about 15 min, concurrence with magnetic pulsations with amplitudes of several tens of nanotesla, horizontal scales of 300-400 km, and occurrence of a pulsating aurora in a diffuse aurora after the passage of the EEASs. Furthermore, the EEASs showed similar temporal evolution to the omega bands, during which eastward-propagating auroral streamers occurred simultaneously in the poleward region, followed by the formation of north-south-aligned auroras, which eventually connected with the EEASs. Thus, we speculate that EEASs may be related to the generation process of omega bands. (Continued)

physics.space-ph↗