Seismic observations in volcanically active calderas are challenging. A new cabled observatory atop Axial Seamount on the Juan de Fuca ridge allows unprecedented real-time monitoring of a submarine caldera. Beginning on 24 April 2015, the seismic network captured an eruption that culminated in explosive acoustic signals where lava erupted on the seafloor. Extensive seismic activity preceding the eruption shows that inflation is accommodated by the reactivation of an outward-dipping caldera ring fault, with strong tidal triggering indicating a critically stressed system. The ring fault accommodated deflation during the eruption and provided a pathway for a dike that propagated south and north beneath the caldera's east wall. Once north of the caldera, the eruption stepped westward, and a dike propagated along the extensional north rift.

The recent volcanic history of Axial Seamount: Geophysical insights into past eruption dynamics with an eye toward enhanced observations of future eruptions.

Following the installation of the Ocean Observatories Initiative cabled array, the 2015 eruption of Axial Seamount, Juan de Fuca ridge, became the first submarine eruption to be captured in real time by seafloor seismic and acoustic instruments. This eruption also marked the first instance where the entire eruption cycle of a submarine volcano, from the previous eruption in 2011 to the end of the month‐long 2015 event, was monitored continuously using autonomous ocean bottom hydrophones. Impulsive sounds associated with explosive lava‐water interactions are identified within hydrophone records during both eruptions. Explosions within the caldera are acoustically distinguishable from those occurring in association with north rift lava flows erupting in 2015. Acoustic data also record a series of broadband diffuse events, occurring in the waning phase of the eruption, and are interpreted as submarine Hawaiian explosions. This transition from gas‐poor to gas‐rich eruptive activity coincides with an increase in water temperature within the caldera and with a decrease in the rate of deflation. The last recorded diffuse events coincide with the end of the eruption, represented by the onset of inflation. All the observed explosion signals couple strongly into the water column, and only weakly into the solid Earth, demonstrating the importance of hydroacoustic observations as a complement to seismic and geodetic studies of submarine eruptions.

During the time period 1970-early 1973, 13 instances of major abrupt depletions of localized regions of the inner solar corona (1.1-2 Rq) were detected at Mauna Loa, Hawaii. Because of their almost invariable association with the ascending Ha prominence material (most generally flare sprays and, in one instance the disparition brusque of a large filament near the solar limb) and close correlation in time and position with outward moving Type IV radio sources, we surmise that the coronal material was expelled from the sun. In several of these cases plasma clouds were tracked to 10 Rq by Naval Research Laboratory with a coronagraph aboard OSO-7. Limited evidence suggests that some aspects of a coronal disturbance, including trajectory of flare spray and depletion of the inner corona, are homologous within 24 hours, so the overall coronal magnetic field configuration is not necessarily permanently altered.

Observations of the white light corona were made on over 900 days during the years 1964-67 at heights between 1.125 and 2.0R~ with the K-coronameter at Mount Haleakala and Mauna Loa, Hawaii. The brightness distribution of the minimurn corona was elliptical with average equatorial intensitics three times the polar. Coronal features of the new cycle at 1.125 R o occurred predominantly in the sunspot zones at 25..-30 ' latitude and in a high latitude zone which migrated toward the North pole before solar maximum. The brightness of the inner corona doubled over this period and a close association is found between the average corona and 10.7-cm solar radio flux. Electron densities in the equatorial regions were nearly twice those of Van de Hulst's model corona, in agreement with the results of recent eclipse observations.