Most organisms maintain a nearly 24-hour circadian rhythm which allows them to anticipate daily events in the Earth’s solar day. Circadian clocks can be regulated by external time cues such as light/dark (LD) cycles, allowing nocturnal organisms to synchronize their rhythms around dusk in a process known as entrainment. In the absence of external time cues, these rhythms persist, revealing the period of the organism’s internal clock. In nature, day-night cycles have gradual transitions at dawn and dusk, yet these transitions often are not reflected in laboratory studies. Most utilize instantaneous transitions from light to dark, including previous work on spider circadian behavior. To mimic natural conditions, the next logical step is to evaluate how a gradual, dusk transition affects activity patterns and entrainment in nocturnal spiders. Two spider species were used: Pholcus phalangioides and Metazygia wittfeldae. Three hypotheses were identified: 1) spider circadian behavior is unaffected by the type of dusk transition; 2) spider activity entrains to a proportional decrease in light intensity; and 3) spider activity entrains to a specific dimness threshold. Spiders were placed in activity monitors and exposed to 12:12 LD cycles with high (8934 lux) and low (2918 lux) light intensities. There were two groups within each light intensity: one with instantaneous, dusk transitions and one with linear 4-hour transitions. In all experiments, spiders were switched to constant darkness after 5 days of LD cycles to identify the phasing of the circadian clock. In P. phalangioides, there were no significant differences among onsets of activity for high and low light intensities with ramped transitions, nor high and low intensities with instantaneous transitions. The average onset of activity was 0.2 ± 0.1 hours after absolute darkness. For M. wittfeldae, onsets of activity were significantly earlier in ramped than instantaneous LD cycles. However, there was no observable difference in activity onset associated with absolute light intensity for either transition type. The average onset of activity for ramped transitions was 1.9 ± 0.5 hours before complete darkness, while the average onset for instantaneous transitions was 0.5 ± 0.1 hours after complete darkness. Pholcus phalangioides requires complete darkness before activity begins, supporting the hypotheses that either the activity onset is unaffected by the type of dusk transition or requires a very low threshold of light. However, M. wittfeldae began activity about 2 hours earlier or around when light decreases by 50%, supporting the hypothesis that this species entrains to a proportional decrease in light intensity. These activity patterns match their ecologies, as P. phalangioides typically sits and waits in a permanent web while the orbweaver M. wittfeldae begins building a new web during the dusk transition. Overall, these results suggest that ramping light cycles should be considered when circadian rhythms are evaluated in the lab.

By examining continuous data from the Poker Flat Incoherent Scatter Radar (PFISR) in Poker Flat, Alaska, short-lived enhancements in the F-region ion temperature, or "Tᵢ spikes", were discovered in the evening while the radar was on the equatorward edge of the high latitude convection region. These enhancements were several hundred Kelvin above the background temperature, would last less than 15 minutes and were preceded by sharp depletions in plasma density (of roughly one half). Though they were mostly detected in the summer, 25 events throughout a whole year of data were identi ed in which the spike occurred within 1.5 hours of the density drop. By examining the location of PFISR at the time of the enhancements, as well as the conditions under which these spikes occurred, it was concluded that these enhancements were the result of electric elds increasing the frictional heating between ions and neutrals. By then examining geophysical data, it was found that these events were temporal and related to changes in magnetic indices. One possible explanation for the observations is that the electric eld is at its strongest near the plasmapause during substorms. Another more likely possibility is that during substorms the region of sunward ion convection expands into a region in the evening side where the neutral gas moves in a direction opposite to the ions, thereby enhancing the frictional heating rate.