In this study, we analyze the first conjugate observation of auroral finger‐like structures using ground‐based all‐sky cameras and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites and investigated associated physical processes that are a cause of auroral fragmentation into patches. Two events are reported: one is a conjugate event, and the other is a nearly conjugate event. The conjugate event was observed at Narsarsuaq (magnetic latitude: 65.3°N), Greenland, at 0720–0820 UT (0506–0606 LT) on 17 February 2012. Analysis of the event revealed the following observational facts: (1) variation of parallel electron energy fluxes observed by THEMIS‐E shows a correspondence to the auroral intensity variation, (2) plasma pressure and magnetic pressure fluctuate in antiphase with time scales of 5–20 min, and (3) perpendicular ion velocity is very small (less than 50 km/s). In the latter event, observed at Gakona, Alaska, on 2 February 2008, the THEMIS‐D satellite passed across higher latitudes of finger‐like structures. The data from THEMIS‐D also showed the antiphase fluctuation between plasma pressure and magnetic pressure and the small perpendicular ion velocity. From these observations, we suggest that the finger‐like structures are caused by a pressure‐driven instability in the balance of plasma and magnetic pressures in the magnetosphere.

No abstract

Distributions of plasma and magnetic pressures are the basic information to investigate macroscopic dynamics of the Earth's magnetosphere. Several studies have been made on magnetic and plasma pressures and macroscopic plasma instabilities in the magnetosphere. However, correlation between magnetic and plasma pressure variations has not been statistically investigated. In this paper, we analyze the statistical characteristics of the phase relationships between variations of magnetic and plasma pressures at frequencies of 4-15 mHz using 2 years of the THEMIS-E satellite data in the nightside magnetosphere. Spectral peaks with coherence greater than 0.85 between magnetic and plasma pressures for 1-hr time segments were selected. The average occurrence rates of the phase relationships are antiphase (within ±10 ∘ from 180 ∘ ), 39.75%; in-phase (within ±10 ∘ from 0 ∘ ), 0.73%; and other phases (10-170 ∘ ), 49.83%. For the other-phase events, the phase differences are much closer to antiphase rather than to in-phase. Thus, we conclude that the two pressure variations tend to be antiphase. The antiphase and in-phase relationships are observed mainly at radial distances outside 8 R E and inside 8 R E , respectively. The high occurrence region of antiphase relationship is in the dawnside during magnetically quiet times and shifts to dusk side at active times defined as Dst < −10 nT. The occurrence rates of the phase relationships do not change significantly depending on the AE and Dst indices, plasma , and IMF-B z . Based on these results, we discuss the correspondence between the phase relationships and the possible magnetohydrodynamic force balances that can produce these phase relationships.Plain Language Summary Distributions of plasma and magnetic pressures are the basic information to investigate macroscopic dynamics of the Earth's magnetosphere. Several studies have been made on magnetic and plasma pressures and macroscopic plasma instabilities in the magnetosphere. However, correlation between magnetic and plasma pressure variations has not been statistically investigated. In this paper, we analyze the statistical characteristics of the phase relationships between variations of magnetic and plasma pressures at frequencies of 4-15 mHz using 2 years of the THEMIS-E satellite data in the nightside magnetosphere. Spectral peaks with coherence greater than 0.85 between magnetic and plasma pressures for 1-hr time segments were selected. The average occurrence rates of the phase relationships are antiphase (39.75%), in-phase (0.73%), and other phases (49.83%). For the other-phase events, the phase differences are much closer to antiphase rather than to in-phase. Thus, we conclude that the two pressure variations tend to be antiphase in the nightside magnetosphere. We also investigate relation of these phase relationships with geomagnetic activities, solar wind parameters, and local times. Based on these results, we discuss the correspondence between the phase relationships and the possible magnetohydrodynamic force balan...

Auroral finger‐like structures appear equatorward of the auroral oval in the diffuse auroral region and contribute to the auroral fragmentation into patches. A previous report of the first conjugate observation of auroral finger‐like structures using a Time History of Events and Macroscale Interactions during Substorms (THEMIS) Ground‐Based Observatories camera and the THEMIS‐E satellite at a radial distance of ∼8 RE showed antiphase oscillations of magnetic and plasma pressures in the dawnside plasma sheet. In the present study, we report another simultaneous observation of auroral finger‐like structures at Gillam, Canada, at ∼0900 UT (0230 magnetic local time) on 14 November 2014 with the Radiation Belt Storm Probes satellites at 5.8 RE in the inner magnetosphere. From this simultaneous observation event, we obtained the following observations. (1) Auroral finger‐like structures developed poleward in the equatorward moving auroral arc at the equatorward edge of the auroral oval. (2) Both the electron and ion OMNI fluxes measured by HOPE increased at ∼0900 UT as the satellite footprint entered the auroral region, indicating that the satellite was crossing the observed auroral finger‐like structures. (3) The absolute value of magnetic pressure was several times that of the plasma pressure, and no systematic phase relationship was identified between the magnetic and plasma pressures, unlike that in the THEMIS case. Based on these observations, we discuss two possible causes of the observed finger‐like structures, namely, pressure‐driven instability in the magnetosphere and gradient drift instability in the ionosphere. In this paper, the latter possibility is newly suggested to develop in the equatorward moving aurora associated with the westward electric field in the equatorward ionospheric density gradient.

Recent Transition Region and Coronal Explorer observations of active loops in the EUV showed that there is a class of EUV loops that consists of near-isothermal loop threads with substantially smaller temperature gradients than are predicted by the uniform loop heating model. These results support coronal heating mechanisms operating in or near the chromosphere and transition region. We propose a new local heating model of loop footpoints in the chromosphere in which there occurs nonuniform heating by slingshot magnetic reconnection during two loops' interaction, driven by the moving solitary magnetic kink, which was recently found by three-dimensional MHD simulation and is characterized as a moving solitary magnetic flux ring with a pair of counterrotating vortex rings. We also show that the loop interaction results in the formation of helical up-and downflows driven by the slingshot magnetic reconnection.